Peter and Greta set up housekeeping and purchase a wedding ring. They hold a wedding reception at Peter's apartment and just before Mr. Brahms and the guests arrive his girlfriend Marilyn appears. Peter whisks her upstairs to Greta's apartment via the fire escape. The man-in-the-middle is watching a romance movie on TV but eventually shuts it off in favor of the fire escape action. Peter and Greta successfully pull off the reception despite a few small slip-ups and a drunken Marilyn showing up, who they pass off as Peter's sister. Greta leaves to go on a date but Peter's mother calls, she's just heard he was married and wants to meet Greta which sends Peter up the fire escape calling for Greta.

Peter's mother sees Greta kissing another man and confides in Mr. Brahms, who hires detective Will Yates. Peter's secretary Miss Wilson tips him off to the situation. Greta refuses to give up her date that night so they go to a movie where they plan to ditch Yates, who sits behind them. The next day he goes to Greta's apartment and tells Greta he knows the truth and wants money to buy his silence. Peter finds out Yates has a jealous wife and he and Greta try to get Yates in a compromising photo. This involves a last minute discovery that Peter's camera is out of film and a druggist who has a difficult time counting. The man-in-the-middle views the action while playing poker with a friend who cheats.

Peter and Greta are invited to the Brahms' country house in Connecticutt for the weekend along with backstabbing office rival Wally Frick and his wife Vera. Peter sees an opportunity to discuss an ad campaign with Mr. Brahms but Greta has a date with Bernie, which she reluctantly breaks. Wally and Vera do their conniving best to foil Peter, meanwhile Bernie has found out where Greta is and shows up at the Brahms'. Greta and Peter let him in a bedroom window and tell him Peter is her brother who has violent tendancies and that they are really at a rest home. Bernie leaves and the others hear an intruder. Figuring it's Bernie, Peter rushes out, makes it sound good and sends Bernie on his way. He succeeds in impressing Brahms but later faints when he finds out the police caught the real prowler, an escaped convicted murderer

Peter accepts a job with another company where his advancement won't be hindered by being a bachelor. He writes a scathing letter to Mr. Brahms exposing the entire setup with Greta. He then goes home and tells Greta their occasional marriage is over. Peter finds out the next day that the man who hired him has been let go, so he's not hired. Peter goes through the mail in Mr. Brahms' office but doesn't have time to get his letter back so he ends up hiring a shady character to break into the office with him and retrieve it. He then has to renegotiate the terms of Greta's employment before she'll agree to continue their ""marriage"".

Peter's mother wants them to re-enact their ""marriage"" in a church so Greta and Peter hire an actor to ""marry"" them. Peter's mother also brings along her own minister just in case, as well as inviting Mr. and Mrs. Brahms and others to the ceremony. The actor, Frank Mastin, gets stage fright and is late, leaving worries that the real minister might perform the wedding. Mastin's agent reminds him of the animated movie he did and after he starts the ceremony he forgets his lines but continues with the ones from the movie in which he married Mr. Bear and Miss Antelope. Peter's mother loves the service and hires the ""minister"" to perform a future wedding for Peter's cousin. The man-in-the-middle tries to hang a picture as the action ensues.

Mr. Brahms summons Greta to his office in order to surprise her with the news he is promoting Peter and giving him a raise and asks Greta to have Peter call him later that evening. Greta tries to do so but Peter is busy romancing Ginger Snap in his apartment and ignores her attempts. Peter finally gets the message after again renogiating his deal with Greta. The next day at Peter's new office Ginger shows up looking for an earring and gets the job as Peter's secretary. Then, in walks Greta and Ginger finds out he is married. Ginger and Greta become friends and Peter is literally left holding the grocery bags. On one fire escape trip Peter and Greta pause long enough to each take a sip from the man-in-the-middles coffee cup.

Peter and Greta both have dates planned when Mr. Brahms shows up unexpectedly with his niece Amanda, asking Peter if she can stay overnight. Peter postpones his date with Cookie and Greta delays hers. Amanda suddenly doubles over in pain. Peter calls the doctor and, after paying the fee, finds out there is nothing wrong. Amanda admits she wants to be actress. She has also figured out Peter and Greta are not married and tries to blackmail Peter into paying for acting lessons. Mr. and Mrs. Brahms arrive and Amanda tells them the entire story. Peter and Greta think they are doomed until Mr. Brahms says Amanda is a pathological liar. Cookie arrives while Peter and Greta are dining on steak and hits Peter with her handbag. The man-in-the-middle is seen dressed in a tuxedo preparing to go out.

Greta needs to supplement her income and takes a job at an employment agency. Her boss Gordon asks her to dinner. Meanwhile, Peter, Wally Frick and their wives have been asked by Mr. Brahms to help entertain an out-of-town buyer. Greta will not break her date so Peter tells Mr. Brahms she is sick and he says Peter should stay home. Instead Peter takes Marilyn to an expensive restaurant where they run into Greta and Gordon. Marilyn and Gordon hit it off and are lost in a world of their own. The Brahms' walk by Peter and Greta's table and are glad to see Greta has recovered. They join the Brahms, Fricks and guest buyer and through charming diplomacy Peter manages to stick Wally with the dinner check while garnering Mr. Brahms' thanks. The man-in-the-middle is seen trying a hair restoration treatment.

Greta is taking pills due to an illness she is recovering from and faints while on a date with Bernie. Bernie rushes her to the hospital and registers her as Greta Patterson. Meanwhile, Peter, Mr. Brahms and Wally are working at the office. Peter is called and Mr. Brahms decides he and Wally will accompany Peter to the hospital. In addition to the name and identity mix-ups for Peter and Greta, Peter must also keep Mr. Brahms and Wally away from Bernie. This leads to some interesting developments, one of which is Peter's story about loose elevator grunches.

Peter and Mr. Brahms are relaxing with a drink after work when Mr. Brahms sees Greta on the dance floor with another man. He decides Peter and Greta need marriage counseling. Figuring a real counselor would see through them Peter and Greta decide to lie and tell Mr. Brahms they've been to a counselor and everything is worked out. Then co-worker Fred arrives at the office bemoaning that his wife just kicked him out. Mr. Brahms wants Peter to set Fred and his wife up with ""his and Greta's"" marriage counselor. So Peter gets his Uncle Harry, a Bohemian artist who dislikes ""manufacturers"", to play their marriage counselor at a meeting with Fred, his wife and Mr. Brahms. The man-in-the-middle is seen opening a beer can.

Peter is asked by Mr. Brahms to help the wife of an important client get her daughter Nancy into acting. It becomes obvious that Nancy has no talent or ambition for acting and is only being pushed by her mother. Peter takes Nancy to visit an off-Broadway producer friend to discourage her. The producer, however, sees dollar signs and allows Nancy's mother to back his play. Peter helps Nancy realize she has no talent and she pretends to be a tempermental actress in front of her mother who finally realizes that Nancy has no desire to become an actress. The man-in-the-middle does exercises in his bathrobe.

Mr. Brahms decides to have a tax filing party at work. Peter signs his and says he will take it home for sick, bed-ridden Greta to sign. Mr. Brahms sends Peter home to care for her and then shows up with flowers. Greta is forced to sign the return and then Mr. Brahms takes it and mails it. Peter and Greta pose as filmmakers to talk to an IRS agent to get ideas on how to retrieve their return, saying it's a movie plot. They finally go back to the IRS, a different agent, and partially telling the truth, manage to substitute Peter's real tax return for the bogus one. Mr. Brahms tells Peter the next day that he has decided to make the tax filing party a yearly benefit.

Greta interrupts Peter's date with Linda to inform him she intends upon marrying Italian Count Umberto Ugazzi, who is involved with one of her art classes. Peter concocts a story that Greta is missing for Mr. Brahms who sends a policeman, Sgt. Krock, to investigate. Complications arise when Linda shows up while Sgt. Krock is there and then so does Mr. Brahms. Krock is about to run them all in when Greta shows up explaining she was suddenly called away to help her sister avoid marrying a man who wouldn't be good for her. Greta later explains to Peter she found out the Count wanted a dowry from her father and that no man in his family had worked for 1100 years!

Peter and Greta are set to go to dinner with Mr. and Mrs. Brahms when Greta's parents show up unexpectedly with her high school boyfriend Arnie Stoner. They insist on taking her to a different restaurant. To get out of the dinner and ward off any advances from Arnie, Greta and Peter tell them they are engaged. This results in Peter being invited along too. Peter gets the Brahms to change their reservation to the same restaurant. Luckily there are two dining rooms at this restaurant so Peter makes the Brahms reservation for the Indian room as the Patterson's are in the Pilgrim room. Peter and Greta spend a frantic evening shuffling between the two tables and eating two meals during which they break off their phony engagement. The man-in-the-middle, who is cooking, is seen when Peter is locked out of his own apartment and goes through the man-in-the-middle's apartment to get to the fire escape.

Greta needs more money and decides to start a dressmaking business with a couple of girlfriends - in Peter's apartment. Peter also becomes the financial backer. The situation escalates when the girls make labor demands of Peter and go on strike. Meanwhile a young wheeler-dealer named Rod McHew wants to merge companies so he can use the baby food company as collateral for a bogus business scheme. Peter becomes suspicious, turns down a bribe from McHew to keep silent and advises Mr. Brahms against the deal in a meeting at his apartment. The strike is resolved when Greta admits she can't get any buyers for the dresses. Brahms thinks the dress factory has been done to make ends meet and gives Peter a raise. Greta's Aunt Harriet has been visiting looking for excitement but leaves saying New York was very peaceful. The man-in-the-middle is sick and doctoring himself during the fire escape scene.

Because she has broken so many dates with Bernie being Peter's occasional wife Greta forces Peter to go to dinner with her at Bernie's mother's, causing Peter to postpone his date for a night with the beautiful Ingrid. While at the dinner Bernie's sister Bernice falls for ""Greta's brother"" - Peter. Now Peter must figure out a way to let her down gently as Bernie warns she has gone and caused trouble at the offices of suitors who rejected her. Peter's secretary Miss Wilson arrives at the lobby of Peter's building with an important business report while Peter is talking to Bernie. She knows him as Peter Christopher while Bernie knows him as Patterson. Peter tells Bernie he is leading a double life and is secretly married. This leads to showing Bernice his wife - Ingrid - and, of course, Mr. Brahms shows up during their meeting.

While dining with Mr. and Mrs. Brahms Greta has her palm read by Madame Celeste who predicts she will marry soon. Although informed she is already married Madame Celeste insists her professional reputation is on the line. An argument ensues which results in Madame Celeste spilling food on Mr. Brahms, who gets her fired. Madame Celeste has her son, who she is putting through law school, represent her. He wants a marriage license from Peter and Greta, then Madame Celeste will apologize. In the meantime, Madame Celeste has organized gypsies to picket Brahms Baby Food. Brahms orders Peter to stop the strike. Peter does so by convincing Madame Celeste to do TV commercials predicting a long happy life for every baby that eats Brahms Baby Food.

Mr. Brahms wants to land a contract with prospective client Ellen Hanley that will put Brahms Baby Food into 1,406 more stores. He orders Peter to invite her to his house for a home-cooked meal. Greta refuses to participate as she has invited a famous French fashion designer from one of her art classes to her apartment for a home-cooked meal. Peter doesn't tell Ellen he's married and Greta's French designer turns into a wolf. Madness reigns. One scene has Greta ushering Mr. and Mrs. Brahms and the designer down the fire escape for the building's ""fire drill"". The man-in-the-middle is painting the trim around his fire escape window but is distracted by the action.

Peter accepts a job with Barnaby Baby Foods, resigning from Brahms and firing Greta. He then finds out that Barnaby has checked him out and wants to meet Mrs. Christopher at a dinner with he and his wife. Greta refuses to go so Peter hires an actress, Sally. Greta changes her mind after discovering Peter has had her phone and utilities shut off. Peter suddenly finds himself at the restaurant with the Barnabys and two occasional wives. Mrs. Barnaby thinks Greta is Peter's wife while Mr. Barnaby believes Sally is. Can Peter keep them from finding out the truth? The man-in-the-middle is washing the blinds on his fire escape window.

Peter and Greta are asked to compete against Wally and Vera Frick for membership in N.O.O.K., the National Order of Kangaroos, by Mr. Brahms, who is King Kang of Hutch #1. The competition is a treasure hunt. The two couples are told at the Kangaroo meeting to leave all their money behind and to obtain all the items by ingenuity alone. Peter and Greta manage to get all the items as Peter trades some of his clothes for them. Wally and Vera cheat, using money that Vera hid. The directions guide Peter and Greta to a car they are to use to transport themselves and their treasures back to Kangaroo headquarters but Wally and Vera tell a policeman they are stealing the car. All four end up at the Police Station. Greta calls Mrs. Brahms and she and Max get everything straightened out.

Greta has been summoned to the office for an executive's wives meeting by Mrs. Brahms. At first she wants a new hat for $23 but Peter bargains her down to $10 and 6 long distance minutes. Peter hires new secretary Cynthia and takes her to lunch. There they are seen from across the room by an important Brahms client Mr. Cartwright who assumes Cynthia is Mrs. Christopher. Greta arrives at the office shortly after Peter returns from lunch with Cynthia, who has been affected by the lunchtime martinis. Mr. Cartwright stops by and mentions to Peter he saw them at the restaurant. Peter madly juggles the two ladies around to keep his secret. When she finds out he's married Cynthia quits. Greta gets both the hat and the $10, which she uses to take a broke Peter to lunch. The man-in-the-middle is playing solitaire.

Peter finds out Mr. Brahms is taking Wally and Vera Frick with him to a convention in Dayton, Ohio to land a prospective client and wrangles an invitation for himself and Greta as there is a $500 bonus involved. Greta is not thrilled at becoming a ""Brahmsette"" until the bonus is mentioned as Dayton is her home town and she might run into family or friends there. While on an errand for Mr. Brahms she runs into her family minister. Parson Perkins is worried about Greta being at the convention until they have a reassuring talk. Mr. Haver, the prospective client, is a very prim and proper person. Peter convinces Wally and Vera Haver is a swinger. Their attempts to lure him to Brahms annoy Mr. Haver but Peter shows up just in time with Greta and her ""personal parson, Parson Perkins"". This lands the client and Mr. Brahms decides that Greta deserves the entire bonus.

Peter wakes up after a night out to realize he became engaged to heiress Felicia Ferris. Brahms is in debt to her father's bank so Peter knows he must break off the engagement without angering her father. Felicia tells him most of her previous boyfriends live out of the country. When she brings a huge sculpture to Peter's office Mr. Brahms demands an explanation. Peter explains Felicia is in therapy and has fantasized they are engaged. Peter attends Felicia's group therapy session and confesses his fake marriage, which no one believes. The doctor tells him he needs help until Peter says he has no money and then the doctor says he is cured and that he and Felicia were not meant for each other. Peter then hears that Felicia and the doctor are engaged and sends them the huge, iron sculpture as a wedding present.

Peter receives a telegram that his foster daughter from Italy is coming to stay with him. Not knowing he had a foster daughter he receives an explanation from Mr. Brahms that he is in a company sponsored ""voluntary"" foster program that is being deducted from each paycheck. Wally Frick sees it as a chance for an ad campaign. Peter's foster daughter Renata shows up and is 18, not the 10 year-old they all expected. She has come to marry Steve, an American tourist she met in Rome, and wants her Papa's blessing. At first Peter forbids the wedding as he doesn't think Steve is concerned about getting a job. He then learns that Steve's father own 312 grocery stores and when Mr. Brahms finds out he worries about keeping his baby food in those stores. With Greta's help Peter apologizes and agrees to let them marry.

Mr. Brahms hires the Chubbs, father and son management consultants, and Wally Frick sets out to get rid of Peter, double-crossing him every chance he gets. Brahms wrote a report on the company's policies, state of business and problems that Peter can't find. Peter invites the Chubbs to dinner, which keeps being interrupted due to Greta's date with Ted. Greta, who has cooked two dinners, is followed down to Peter's apartment by Ted and Inez, one of Peter's girlfriends, arrives at the same time. The next day at the office the Chubbs are ready to finish Peter off. Chubbs, Jr. pulls out the missing report, believing Peter wrote it, and pronounces it rubbish. Mr. Brahms coolly dismisses the Chubbs and calls for Wally, telling Peter hiring the consultants was Wally's idea.

Brahms Baby Food has signed a contract to supply powdered milk to the government and all employees must undergo a security clearance check. On the home front Greta is informed the landlord wants her apartment for his newly-married niece. The ISA men visit Peter's apartment and then interview the man-in-the-middle and Greta in her apartment. Peter and Greta convince the landlord that it would cost him too much to fix the apartment up for his niece and he agrees that Greta can continue living there. The ISA men return and say they suspected Greta was posing as Peter's wife but after talking to many witnesses realize it's a very modern marriage and that the apartment upstairs is her art studio. Having passed the security clearance Peter and Greta celebrate. Peter finds out the next day Brahms has signed a business deal with West Germany and informs Peter the German security checks are very thorough.

During Greta's party in Peter's apartment Mr. Brahms calls and talks to ""Greta's boyfriend"". Greta tells Mr. Brahms it was Peter's brother playing a joke. In honor of Peter's brother being in town the Brahms' invite the three of them over for dinner. Peter hires his milkman Teddy, who is tired of his job, to play his brother. They pass him off as a writer who is leaving on a tramp steamer in the morning. Mr. Brahms offers Teddy a job to keep Peter's family together. Teddy accepts and then Peter and Greta conspire to drive him out of Peter's apartment by convincing him Peter is a little insane and dangerous. Mr. Brahms complains the next day that Teddy quit and took two of his secretaries with him. Peter replies that it's a burden he's had to live with all his life.

Bernie is insistent that Greta marry him so Peter tells him she is seeing another man. He confides in sister Bernice who encourages him not to give up. Mr. Brahms has asked Greta to help him shop for an anniversary present for Mrs. Brahms. Bernie and Bernice see them and assume Mr. Brahms is Greta's sugar daddy. Then they see Greta dressed in mink meeting Mr. Brahms at a bar. Bernie hears where they are going to meet, a hotel room that has actually been rented for a surprise party, and gets there first. He attacks Peter when he enters and Bernie is knocked out in the struggle and hidden in the closet. Greta and Peter spend the evening guarding the closet. Bernie later pops out singing ""Happy Anniversary"". The man-in-the-middle is having a party.

Peter has invested in an oil well scheme with Col. Thorndyke and his daughter Linda Sue which Mr. Brahms is suspicious of. He surprises Peter at Thorndyke's hotel and insists on investing when Peter tells him Thorndyke is in no hurry for the money. Greta and Mrs. Brahms have Thorndyke checked out and learn he is honest but has never had a well come in. Peter is ordered to get Mr. Brahms' money back or else but fails to. Greta appeals to Linda Sue, who was romantic with Peter before finding out he was ""married"". She arranges for both Peter and Mr. Brahms to get their money back. Shortly afterward they learn that the well did come in. Greta does her best to console Peter by waiting on him. When a girlfriend drops by she offers to leave but Peter announces Greta is his date for the night.

Peter's mother calls him while he's with a date and announces she's coming over to stay for a while because of the paint fumes in her apartment. They say they have dinner plans and she plays the martyr, telling them to go out. She then inspects the apartment and takes a phone call from one of Peter's girlfriends, informing her Peter is out with his wife. She has Greta exhausted doing housework during the day and Peter's social life is in ruins. Peter suggests they need to be alone and his mother mentions her doctor and implies she doesn't have long to live. Peter checks with her doctor who says his mother is a hypochondriac. She finally packs to leave and as Peter kisses her goodbye she can feel he has a fever. She decides she will stay and nurse him back to health. The man-in-the-middle attempts to set up a sun chair on his fire escape landing.