Mike and Sally finally get married and leave for the East where Mike has a job as an assistant college psychology instructor. Robbie happily moves into Mike's old room. When Uncle Charley notes that they may soon be calling Steve 'Grandpa' he becomes somwehat concerned about growing old and calls up his former girlfriends. Robbie, now a college man, offers to help his father make friends with younger women, and while pretending to be disinterested, tries a few of his suggestions. When it is learned that Ernie must return to an orphanage, because he will not be allowed to accompany his foster parents in their move to the orient, Chip invites Ernie to move in permanently, giving Steve three sons once more.

Steve explores the possibility of adopting 10-year-old Ernie but runs into antagonism from Uncle Charley, who can forsee nothing but more work with a new boy to take care of. The family desperately wants to adopt the boy and Robbie decides that Steve must make a play for the lovely Miss Coulter. Her boss comes to inspect the house just as the boys have made a shambles of the place. At eight the next morning, Miss Miller returns. She inspects the house, spic and span now, and ushers in a happy Ernie, who is to stay on a temporary basis.

Ernie's adoption into the all-male Douglas household is threatened by a regulation that there must be a lady of the house. Steve again makes a request to adopt the child, but Misses Coulter and Miller of the adoption agency, say they are powerless to change the regulations. Miss Miller has a brilliant idea, however, and soon the family is appearing before a judge who decides that housekeeper Uncle Charley can legally be designated the 'lady of the house'. Charley approves if this fact is kept secret and Ernie is enthusiastically welcomed into the family.

Uncle Charley is upset when he learns that Robbie is dating a flashy chorus girl, but Steve refuses to worry because he trusts his son's common sense. Charley predicts Steve will be invited to pay the girl cash on the line if he wants her to stop dating his son.

An escaped circus lion makes a midnight call at the Douglas house, and Ernie sees it and awakens the understandably skeptical Steve who accompanies his new stepson to investigate. Steve eventually sees the lion and nervously plays a game of tag with it, warning the family to keep their bedroom doors closed.

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Chip's secret love is an older girl named Mary Lou. When Steve suggests his love sick son should find out where he stands by asking for a date, Chip calls and identifies himself as 'Douglas'. Mary Lou is delighted -- thinking she is going out with Robbie Douglas, a genuine college freshman.

Ernie writes a play and volunteers Steve's services for his father-and-son night, so he dons a heavy robot costume. The family leaves early, and Steve finds himself alone on the street, helpless in his non-removable suit.

A seductive dance instructor named Helen signs Uncle Charley up for a lifetime membership in a dance club. Dreaming of a professional dancer's career, he purchases a top hat and tails for his appearance at the school's annual ball.

Robbie is so eager to make the track team that, to improve his timing, he agrees to join a ballet class. He proves an unwilling pupil, self-conscious in dancer's tights and ballet shoes, but after days of practising, his hurdling style suddenly improves. Reluctantly, he agrees to become the sole male dancer in the upcoming Ballet Class recital.

Ernie feels neglected when the other members of the Douglas family are busy with various girlfriends and he is left alone. Close to tears, Ernie is puzzled and saddened by the mysterious powers of the opposite sex. He's even more puzzled when he meets eleven year old Linda-Lou.

Steve Douglas turns a business trip to Hong Kong into a vacation for the whole family. Uncle Charley, who once lived in China, is reluctant to visit modern Hong Kong, for apparently he still carries a torch for a beautiful girl he knew in the 1930s.

Steve's unusual behaviour convinces his family that he is about to be secretly married. The boys plan to leave home to provide privacy for the newlyweds, and Charley arranges to ship out on a freighter. Steve brings the pretty blonde girl home to meet the family, only to find the house deserted.

The Douglas family bravely accepts the challenge when 13 year old Chip decides to give a party -- with girls. On the party evening the record player breaks down and Chip contemplates the disintegration of his first teenage party. But Steve comes up with a solution.

When Steve travels to Washington on business, he leaves $50 with Robbie, to pay for golf club repairs but Uncle Charley limbers up his billiard cue to get revenge after two girl pool sharks take Robbie's $50 when they coyly ask him to show them how to play the game.

A huge shaggy stray dog adopts Steve Douglas and disrupts the neighborhood with his howls when Steve tries to leave home. A neighbor and lover of stray animals, is delighted to learn that Steve plans to keep the dog until its owner can be found, rather than sending it to a shelter. However, the dog continues to regard Steve as his mother as well as his master.

Steve Douglas meets recently divorced Maggie Bellini, reputed to be one of the world's ten richest women. She sets her sights on Steve by showering his family with costly gifts. Robbie has the use of an experimental sports car, Chip is invited to fly to Austria for a weekend of skiing, and Ernie is provided with the use of a computer for his arithmetic homework.

When Robbie Douglas yanks pretty co-ed Terri Wong out of the path of a speeding truck, he becomes the unwilling beneficiary of an old Chinese custom when the girl whose life he saves insists on being his slave. The game is amusing at first but before long Robbie must ask his father for help.

Steve Douglas is attracted to a beautiful female explorer who shows Steve and the family her exciting adventure films. She has to prepare for a safari, and as there is room in the party for one more, she begs Steve to come along.

Robbie takes on a heavy work load of waiting on tables in his girlfriend's sorority house and working as a chemistry laboratory worker, so that he can rent a room away from home. Worst of all, his romance with his current girlfriend languishes because he is too tired to take her on a date.

When Chip adopts a Beatle-style, shoulder length haircut, the other members of the family feel that Steve should lay down the law to the long-haired one, but he and his associate consult a company psychologist for a solution. Steve decides not to be a strict father but to give the boy more attention, without affect.

Robbie Douglas is practically engaged to his girlfriend Joanne, and when they discover how their friends are both successfully attending university while raising a baby they begin to think that perhaps teen-aged marriages are workable.

Two tomboys - a pretty lady engineer named Max and a member of the girls Hockey team named Georgie prove tough to handle for both Steve and Chip. Hoping to become a hero to his peers, Chip goes out for the girls hockey team when Georgie insists the coach must let her join the boy's track team.

Robbie Douglas learns how much of a pest a younger sibling can be when Chip and a buddy break up his date with a pretty girl by eating appetizers and otherwise making a nuisance of themselves. The next afternoon, for revenge, Robbie primes his other brother Ernie to haunt Chip and his date.

Robbie has a wonderful time going steady with two girls simultaneously -- a highschooler and a college co-ed -- until the two ladies compare notes one day and plot immediate revenge. They maneuver Robbie into dating the both of them on the same night.