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Harry and Tacey King enjoy an idyllic life in a pleasant suburb known as Hummingbird Hill.  However, problems arise when their resident maid quits because she can no longer deal with the Kings's three young boys Larry, Tony and Roddy, and the enormous family dog.  Harry, a lawyer, and Tacey as well as another young couple, Bill and Edna Philby, are invited to dinner at the home of Harry's boss and neighbor Mr. Hammond.  They have difficulty finding a babysitter for the evening and have to settle for Ginger, a sixteen-year-old with a crush on Harry.

After dinner, the Hammonds and their guests are visited by their mutual neighbor, the prissy, nosy Mr. Appleton, who has come to deliver a letter he has received in error.  He tells the Kings that he is surprised to see them there as he thought they would be at their own party.  Harry and Tacey rush home and find that Ginger has invited a number of her friends over for a loud dance party.

Because of this incident, Tacey places an advertisement for a resident nanny.  After a series of telegrams are exchanged, the Kings decide to hire Lynn Belvedere, who has an impressive list of qualifications.  While Harry is waiting at the train station for his new employee, who he assumes is a woman, a gentleman comes to the Kings's house and announces that he is Lynn Belvedere, a genius who has mastered many professions.  Although uneasy about hiring a man, the Kings decide to give him a trial for a few days.  Within a matter of hours, Mr. Belvedere has won the children over, fixed the ice box, subdued the rambunctious dog, given the baby a bath and prepared a superb dinner salad.  He soon establishes himself in Hummingbird Hill and proves a worthy adversary to Appleton.  There are times, however, when the genius gets on Harry's nerves.

In addition, Harry is concerned about leaving Tacey and Mr. Belvedere alone together while he goes on a business trip and asks her to sleep at the Philbys' house.  This arrangement works well until one night Tony wakes up with stomach ache and wants his mother.  While Tacey is in her home caring for Tony, Appleton, having noticed the lights on, comes over to see if anything is wrong and finds Tacey and and Mr. Belvedere in their dressing gowns.

The next day, Appleton gossips about what he has seen to the other neighbors and it is eventually reported to Hammond, that Mr. Belvedere was in pajamas, Tacey in a flimsy negligee and both were cavorting about, drinking gin.  When Harry returns home, Hammond, who requires total respectability from his employees, tells Harry what he has heard.  Harry accepts Tacey's explanation, but feels that Mr. Belvedere should leave to avoid further complications.  When the boys protest vigorously, however, Harry reluctantly agrees to let Mr. Belvedere stay.

Sometime later, after attending a lecture on child psychology, Tacey and Edna stop at a hotel restaurant for a snack.  At another table are Mr. Belvedere and Harry's secretary, Peggy, who has brought Mr. Belvedere some papers.  Peggy leaves before she is spotted, then Mr. Belvedere invites Tacey to rumba.  Appleton and his mother, who have also attended the lecture, see the couple dancing and another rumor begins.  Following a big fight with Harry, Tacey takes the baby and goes to stay at her parents' house.  Although Tony asks his father to phone Tacey, both stubbornly refuse to communicate.

Later, the neighborhood is up-in-arms after the publication of a book, Hummingbird Hill by Lynn Belvedere, about the manners and morals of suburbia and which contains thinly disguised portraits of several well-known people, including Hammond, who is depicted as an office lothario.  Tacey hears from Bill and Edna that Harry has been fired for employing Mr. Belvedere and Bill has been fired for defending Harry.  Tacey returns home to find Movietone News interviewing Mr. Belvedere in her living room.  She and Harry reunite but she doesn't know whether to congratulate Belvedere or spit in his eye.

Hammond then arrives and announces that he is suing Mr. Belvedere for libel.  Appleton and the others in the community whose peccadilloes have been revealed in the book also intend to sue.  Mr. Belvedere is delighted by this, as it will result in more copies of his book being sold, and he asks Harry and Bill to represent him in the legal battles.  When Mr. Belvedere identifies Appleton as his main source of information, the others vent their wrath on him.

Later, Harry and Tacey decide to go out to dinner, and are surprised to find that Mr. Belvedere is still there, prepared to continue performing his duties.  After Mr. Belvedere reveals that he plans to produce another two volumes and will write the entire trilogy under one roof, Harry and Tacey tell him they are expecting another child.  Mr. Belvedere assures them that they will find him to be of great help as he is also an expert obstetrician.