The day before her wedding Katie Miller feels she faces complete disaster. When Robbie tries to console her, she flares up at him and after a short argument, the pair declare the wedding is off. Of course, Grandma Collins thinks otherwise.

The Douglases move to California, where at first they are depressed to find the residents as chilly as the weather is warm. Robbie enrols in the University and is totally captivated by a blonde girl, and Uncle Charley is initiated into the rapid-fire procedure of shopping in supermarkets. The family begins to feel that their next door neighbors represent the characteristic stand-offishness of all Californians until they discover that they moved into the neighborhood the day before they did.

Early on his wedding day, Robbie Douglas and his family are peacefully oversleeping. They awaken just in time and scramble madly to get ready as Robbie and Katie are soon married amid traditional confusion and tears. The lost family dog, Tramp, returns in time to join the family in the front pew.

The day -- or rather the night -- of the big event finds expectant mother Katie quite calm, and Steve and Uncle Charley quite the opposite, and a hapless father to be off on Army manouvres. At the hospital, Katie gives birth to identical triplet boys and Steve must adjust to the fact that he is a Grandfather, three times over.

Chip Douglas is so nervous about taking his driving test that his new sister in law Katie decides to help by taking the test with him. Soon after they both get their driving licenses the family car collects a big scratch, but neither will plead guilty.

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.

The usually all-male Douglas household becomes a refuge for an attractive girl dancer who has been evicted from her apartment, when Steve attends an office stag dinner. He takes her home where he lends her some of Robbie's clothes.

Steve Douglas takes his family to his birthplace of Bedford Springs. He tries to locate his 1940 fiancee Ellen, married and still living in town, but he is disenchanted by the changes in her. Still pretty, she lives with her obnoxious husband and ill-mannered children.

Robbie and Katie move into an apartment on the next block and widower Steve Douglas becomes the target of Katie's relentless matchmaking. However, Steve meets a remarkable woman in a high school corridor and the two are quickly attracted to each other.

Everyone (Except Barbara) knows Steve is going to propose to Barbara

Steve and Barbara have different ideas on how their wedding should go.

On Steve's wedding day the groom is predictably nervous. Recalling certain mishaps from his own big day, super efficient Robbie determines that his own father's wedding will be perfect and after a few hours of hectic, last-minute preparations, Steve and Barbara are married in a simple church ceremony.

The town of Bryant Park takes sides violently when it is proposed to cut down the tree which stands outside of Robbie's girlfriend's home. The Douglas family are soon involved as Bub joins a women's group in the neighborhood to save his favorite tree from being torn down.

Steve Douglas gets a parking ticket from Robbie's girlfriend, a pretty meter maid, and another citation for not having his driver's license renewed -- and then flunks his driving test. Soon Robbie realizes that he's given his father last year's manual to study and that some laws have been changed.

Chip invites his neighbour's long-haired, guitar playing boy from Liverpool as a key addition to his off-key rock and roll band. The English lad's soft, folksong guitar is a disappointment and obviously won't work with the raucous amplified trio, but he helps them to win a trophy in an amateur talent contest.

Robbie needs to talk to his dad-who seems unreachable

Steve and Barbara's Honeymoon in a remote fishing village is bustling with hordes of other, younger honeymooners. They end up counselling several couples who are not adjusting to married life. When a bottle of champagne arrives from the family back in Los Angeles, the other honeymooners realise that Steve and Barbara are indeed honeymooners as well.

Chip begins studying with a pretty co-ed whose disciplinarian father wants to know why her new friend is spending so much time with her. Meanwhile, on the eve of Steve and Barbara's first wedding anniversary, Dodie announces she is throwing a surprise party, but neglects to tell everyone when or where.

Robbie's science master announces a special achievement award for being a straight 'A' student and to his embarassment, Robbie wins it. This causes more trouble than he could have imagined but Steve does his best to assure Robbie that he did deserve the award.

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.

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.

The Douglas family returns from a trip to Britain and as the family watches home movies of their vacation, the amateurish footage reveals the reason for Steve's current moodiness: He is still carrying a torch for a lovely widow.

The contractor that Steve hires to add a baby alcove to Robbie and Katie's room leaves much to be desired. He displays all the charm of a top sergeant and is putty in Katie's hands, but is exasperatingly gruff and non co-operative to the males in the Douglas home.

With Steve and Barbara due back from their honeymoon in Mexico, Katie forces Robbie to lecture Uncle Charley on politeness, and the result of his very puzzling behaviour nearly drives Barbara out of her mind.