Based on Gwen Davenport’s novel, a self-proclaimed genius takes a job as a live-in babysitter for a family of five. The wife likes his capabilities so well that the husband becomes jealous.
In the suburban community of Hummingbird Hill a taxi driver stops and asks Clarence Appleton (Richard Haydn) for directions to a local address. Clarence gives him directions and tells how he breeds flowers. His mother Mrs. Appleton with binoculars asks what the man wanted, and Clarence tells her. She wanted him to get more information about the Kings, and he says he tried.
The cab driver goes to the house and rings the doorbell. Two small boys and a great Dane run over, and the dog jumps on him. Tacey King (Maureen O’Hara) answers the door and says no one phoned for a cab. Mrs. Maple comes down the stairs with a bag and tells Mrs. King that she has quit and is leaving because of her three kids and the dog. Mrs. Maple leaves, and the two older boys console their mother.
In his office Harry King (Robert Young) gets a call from his wife Tacey who tells him the maid quit, and she is glad she is gone because she drank beer all day. Harry tells her they need a babysitter for tonight because his boss Mr. Hammond has invited them and the Philbys for dinner. He tells her to get a sitter.
Harry stops his car in the driveway and removes the children’s toys. The dog jumps on his back as he is unloading groceries. He tells the boys he got ice cream.
Harry shows Tacey the groceries in the kitchen, using a foreign accent in fun. They kiss, and she says they cannot afford a maid unless he got a raise. She says she phoned fifteen sitters without success. Harry says he will try and calls several sitters while Tacey comes in the bedroom to get dressed. She suggests he tell Mr. Hammond he can’t get anybody. He does not want to disobey his boss because starting a new law firm is difficult. He calls Mabel, and she says he should drop dead. Harry suggests calling Ginger; but Tacey objects because she has a crush on him. She relents, and he calls the 16-year-old.
Harry in a tuxedo is driving Ginger in the car, and she cuddles next to him. She flaunts her perfume in his face.
In the house Ginger asks the Kings if she can make a few phone calls, and the Kings leave. Outside Tacey asks Harry if he would like to stay home with Ginger, and he kisses Tacey.
At a table the Kings and Bill Philby (John Russell) and his wife Edna Philby (Louise Allbritton) figure out that the Kings won at cards, and the Hammonds owe the Kings $5.20. Edna tells Harry that the Hammonds had better win if he wants a raise. Horatio Hammond (Ed Begley) and his wife come down the stairs, and Harry tells them that they won $4 from the Philbys and $1.20 from the Kings. Horatio goes to answer the door, and Clarence comes in and gives him a letter he says he opened by accident. Mr. and Mrs. Hammond offer him coffee and a sandwich, and Clarence tells the Kings that a loud party is going on in their house.
Six teenagers are dancing to swinging music in the living-room as Harry and Tacey come in. They stop dancing, and the three children come down the stairs and are sent back to bed. Harry tells Ginger to leave with her friends, and Ginger says his wife has a bad disposition.
Clarence is using a feather to gather pollen from iris flowers, but the owner warns him to leave. Clarence sees a man taking out bottles to the curb and comments about his party last night. The man sends him away.
Clarence gathers pollen in the front yard of the King’s, and the two older boys ask him what he is doing. Inside Harry sees him and asks Tacey about it. She gave him permission to gather pollen, and Harry suggests they ask for a stud fee. Outside Clarence takes the mail from the mailman and hands it to Harry.
Inside Tacey tells Harry she got an answer to her ad for a live-in babysitter. She reads the answer from a person who claims to know psychology. Tacey says she will follow it up.
Tacey and the boys are fixing up the maid’s room. Harry comes home and finds an easy chair blocking the stairs. Tacey tells him to climb over it and says they have been busy. She says the babysitter, Lynn Belvedere, is coming tonight on a train. Tacey gets Harry to move the chair which falls on him on the floor.
Later Tacey and Edna are waiting for Harry to return with the babysitter. Bill calls, and Edna says she will be home after the babysitter arrives. Tacey answers the door, and Lynn Belvedere (Clifton Webb) introduces himself. Tacey expected a girl, and Edna laughs. Tacey explains she expected a woman. Belvedere says her ad did not ask for that, and he says he does not like children but is capable of taking care of them. He denies he is there on false pretenses. He asks to see his room and persists until she shows him upstairs. She tells the boys to go back to sleep. She introduces Larry (Larry Olsen) and Tony (Anthony Sydes) to the new babysitter. He looks at the room and says it will be satisfactory after he removes items. He likes simplicity and declares he is a genius. He gives her a towel that says “Hers” on it, and she says she will change it.
Harry comes in and complains she never showed up. Edna and Tacey laugh, and Belvedere appears on the stairs asking the time of breakfast and saying he is a vegetarian. He goes back to his room, and Tacey says that is Lynn Belvedere.
At breakfast Tacey asks Harry what they are going to do about Mr. Belvedere, and he says he will tell him to leave. Harry and Tacey go upstairs as the baby Wally flings cereal at the other boys.
Harry and Tacey see that many things from the room have been put in the hallway. Harry says he is going and pounds on the door. Hearing nothing, they go in and see Belvedere doing a headstand. He stands up and says good morning. He says when practicing yoga he does not see or hear anything because he is out of this world. Harry says he is weird.
At breakfast Harry tells Belvedere this can’t go on, but Belvedere says he will carry out his end of the agreement. He explains he has bathed children of all ages. Harry asks Belvedere about his being a genius. Belvedere says he is a philosopher. Tacey asks if he can handle children. Belvedere says he dislikes children but can handle them if he is given a free hand. Harry goes out, followed by Tacey. Belvedere tells the children to eat their breakfast and chew their food. The baby throws food on him.
Outside Harry and Tacey decide to give him a chance. They hear the baby scream and run inside to find the cereal bowl dumped on the baby’s head. Harry says the baby has thrown food on him, and he approves what Belvedere did.
Harry comes home and finds Larry and Tony doing headstands in the garage. In the kitchen Tacey is cooking and kisses Harry as he comes in. She says the children have been good all day, and Belvedere fixed the icebox. Harry sees the dog lying on the ground, and Tacey says Belvedere was a dog trainer. She shows him a salad Belvedere made. She says he is mysterious and locks himself in his room in silence. He is out for his constitutional, which he does every night before dinner. She says she has an extra key and suggests they look in his room. He says that is unethical but agrees.
They try the key, but Belvedere appears and says he is an expert locksmith and changed the lock. The boys say they are washing their faces and hands for dinner, and Harry says he is sold.
In the winter snow Belvedere is walking with a large package. Clarence asks if he is happy with the Kings. He invites Belvedere to meet his mother, but he declines and walks off.
Tacey and Harry have built a snowman, and Tacey works on the mouth. Belvedere tries to sneak by but is seen. He criticizes the face of the snowman and goes in. Harry wonders what is in the package. They both heard ticking and are suspicious. Harry tells her to watch his door while he climbs up a tree to look in the window. Belvedere sees her sneaking up the stairs and looks out the living-room window and sees Harry. Belvedere goes outside and speaks to Mr. King who falls from the tree.
Belvedere is bandaging Harry’s injured arm and says he is a bone specialist. Tacey sees a bird cage, and Belvedere explains he is grateful for her gift of woolen socks. He will teach the bird to sing.
Tacey is packing shirts for Harry’s trip to Chicago and says Bill will take him to the station. Harry finds the baby in the bathtub and tickles him. Belvedere comes back in and tells him that he is teaching the baby that bathing is not a social function. Harry tells Tacey that the genius gets on his nerves; but she says she is mad about him.
Bill toots the horn, and Tacey tells Harry it’s time to leave. Harry tells the boys to be good and says goodbye. Belvedere says he will pinch-hit for him while he’s gone. Tacey says they will be fine. Harry tells Tacey he cannot leave her alone there with Belvedere because she finds him very attractive. Bill and Edna come in, and Harry says he is not going. Edna offers to have Tacey sleep at their house, and Harry agrees to that. Tacey refuses to kiss him, but Harry kisses her and goes out. Belvedere comes down the stairs and confesses to eavesdropping.
At night Tony has a stomach ache and wakes up Larry who says it is 3 in the morning. Tony says he tried standing on his head, but he threw up. Larry decides to wake up Belvedere who gets on the phone. Larry pours from a gin bottle but explains to Belvedere it is only ice water.
Edna hears the phone and wakes up. She says Tony is sick and calls Tacey.
Tacey comes in and asks Belvedere if he took his temperature. He says he only has a bellyache. She carries Tony upstairs to bed, and he asks if she will sleep there now. She says no, kisses him, and says goodnight. Downstairs Belvedere answers the phone and assures Edna everything is all right. They hear the doorbell, and Tacey opens the door, letting Clarence in. He asks if anything is wrong. He sees Belvedere in his robe with the gin bottle which he pours into a glass and drinks from. Clarence goes out, and Tacey calls him an “evil-minded little worm.” Belvedere suggests he could get a swarm of bees to ruin his irises. She laughs and says goodnight.
At a bookstore Clarence tells the bookseller what he saw at the Kings. Women spread the gossip by telephone, and Mrs. Hammond calls her husband at his office. Horatio says that is outrageous that both were drunk.
Harry returns to his office and is told Mr. Hammond wants to see him. He goes into his office and describes the successful agreement he got in Chicago. Mr. Hammond tells him that while he was gone, his wife and Belvedere were having quite a time.
Tacey is sculpting a bust of Belvedere while he sits in a chair. Edna compliments Tacey on her artistry. Edna goes out, and Belvedere tells Tacey how expert he is on anatomy. He has her put her hands on his jaw bone. As Harry comes in, she says, “Mmm, do it some more.” Harry says it is a cozy scene and asks what they were doing. Belvedere says she was feeling his larynx. Jealous Harry complains about what Hammond told him from the gossip. She tells him to stop ranting, and he admits the rumors are probably garbled. She realizes it was Clarence. Belvedere criticizes Harry for being a typical suburbanite. Tacey says Clarence came snooping around. Belvedere says it is silly. Harry says it would be better if Belvedere left, but Tacey asks if that should be done. Larry and Tony come in and plead for Belvedere. Tony calls him “Uncle Lynn,” and Belvedere tells him never to call him that again. They hear the baby crying, and Belvedere says that children are psychic. Harry decides he can stay and shouts, “Shut up!” The crying stops, and the bird sings.
Tacey and Edna come out of a lecture on child psychology they did not like. Edna suggests they get something to eat there with good music. A young woman tells Belvedere who is there and offers to leave. He thanks her for the notes, and she goes. Edna and Tacey see Belvedere at another table, and he comes over. Tacey agrees to his joining them, and he sits down. Tacey asks him if he dances, and he says he taught Arthur Murray. He asks her to dance, and he dances better than the others. Tacey says he dances divinely, and he agrees. Clarence and his mother come in and see them dancing cheek-to-cheek.
Tacey comes home late at night and tries to tell Harry who is in bed why she is late; but he rolls over and says to tell him in the morning.
Mr. Hammond orders Harry to come into his office and lights a cigar. Harry comes in, and Mr. Hammond stands up and tells him his firm has a spotless name. He does not allow scandals among his employees and refers to what happened last night. Harry asks what it was, and Mr. Hammong begins to tell him.
Harry comes home early and asks Tacey in the kitchen why she did not tell him about her dancing with Belvedere last night. Belvedere is shredding lettuce and says it was delightful. Harry is jealous, and Tacey says he dances badly. Harry says Hammond gave him an ultimatum. She asks Harry to apologize. He asks why he should apologize for her indiscretions. She says she will move with the baby to her mother’s until he comes to his senses. Belvedere says both are acting foolishly, but Harry is worse. She agrees with Belvedere, and Harry complains she always does. She stalks out, and Harry says Belvedere broke up his home. He says he is going to get drunk, and Belvedere says that is often done by the “mentally immature.”
On a porch Tacey talks with her parents. She admits she had a fight with Harry over Belvedere. Her mother sense she may have been in the wrong. She walked out and has not phoned him. Tacey is hoping he will call her.
Harry asks Larry what is wrong, and he asks when Mom is coming home. He and Tony are lonesome for her. He urges his father to call her. Larry asks what a moron is, and he explains. Larry says someone called him that. Belvedere comes down stairs and says it is his evening off. Harry asks if he said he was a moron. Belvedere advises him not to act like one and suggests he call his wife. Harry fires Belvedere who says he will be helpless without him during his wife’s absence. Harry says he has a half a mind to hit him, and Belvedere replies that takes half a mind. Harry tries to slug him; but Belvedere dodges the blow, and Harry hits his fist on the door frame.
Newspapers report the sensational new novel about Hummingbird Hill by Lynn Belvedere. Clarence tries to revive his mother.
Hammond comes into his office and tells his secretary to get him books on libel. The three office women discuss the novel that exposes Hammond who chases girls around the office trying to pinch them. They admit it is true and wonder how Belvedere knew.
At the bookstore the woman tells customers on what pages the book is about them.
Tacey reads in a magazine a review of the satire on Hummingbird Hill by Lynn Belvedere. She gets a call from Edna who tells her that Belvedere has the town in an uproar. Harry was fired, and when Bill stood up for him, he was dismissed too. Edna tells Bill it worked because she hung up.
Tacey with the baby arrives in a taxi and finds people gathered in the front yard. Inside Belvedere tells the director he needs no instructions. They start the interview before the cameras. Larry and Tony run to Tacey and hug her. They go in, and she embraces Harry. The director tells them to be quiet, and Belvedere says, “Cut.” Belvedere tells her that Harry will not starve. The director says they have not finished, but Belvedere disagrees. Edna and Bill welcome Tacey who goes upstairs. The TV people wrap it up, and Belvedere tells Harry his book has made him immortal. Hammond comes in and hands a summons to Belvedere. He and three others are suing him for libel. Belvedere asks Harry and Bill to represent him on the suits. They agree, and Bill says they will start a new firm. Hammond says they will sue others too. Belvedere suggests he go to the source and indicates all the gossip came from Clarence who asks if he collaborated. Belvedere says no but calls him a stool pigeon who should be roasted. Hammond and the two others chase Clarence out of the house. Harry and Tacey laugh about that, and she sits in his lap. He suggests they go dancing. She asks who will sit with the kids, and Belvedere says he will. She asks his plans, and he says he will write two more novels about Hummingbird Hill. Harry asks if he will write them there, and Belvedere says he will even though he does not like children. Tacey tells him they are expecting another child. Belvedere says he was also an obstetrician.
This comedy portrays an eccentric but fantastically accomplished man showing how he can be an outstanding babysitter and handy man because he practices spiritual disciplines such as yoga, meditation, and vegetarianism and has fantastic amounts of education and experience.