A widower and aeronautical engineer named Steven Douglas raises three sons with the help of his father-in-law, and later the boys' great-uncle. An adopted son, a stepdaughter, wives, and another generation of sons join the loving family in later seasons.
Steve's look-alike Scottish nobleman cousin Fergus, comes to America looking for a bride to take back to his hometown of Sithian Bridge and moves in on the entire Douglas family, much to the dismay of Uncle Charley.
When Uncle Charley learns that Fergus is looking for a wife to take back to Scotland to provide him with heirs to carry on the family name, he takes out an ad in the local newspaper in an effort to rid the Douglas home of his nemesis.
Steve's Scottish cousin Fergus, after only one date with Bowling Alley Cocktail Waitress Terri Dowling, proposes but gets turned down by her. She feels inadequate to return to Scotland as royalty, but is eventually persuaded by Steve.
The wedding day for Steve's Scottish cousin and bowling alley waitress Terri Dowling finally arrives amid the traditional confusion and tears. They leave for a three day honeymoon in Las Vegas but Uncle Charley is worried about the lack of thankyou from them.
Having no choice, Steve takes the triplets to work where a secretary offers to take them home until he is finished. However he accidentally loses her details and when she finally rings, she announces that the boys have run out the front door, much to Steve's annoyance.
Chip complains about Polly's lack of sales resistance because they have an apartment full of useless things. But she is trapped by a salesman into giving a dinner party for eight people in exchange for a free set of cookware.
Uncle Charley leaves for a week's vacation in San Francisco but meanwhile, birthdays disrupt the Douglas household, when Barbara, Katie and Polly turn against their spouses over the men's apparent oversight.
Childless young marrieds Chip and Polly Douglas suddenly find themselvs the temporary guardians of several young children including the triplets, when Katie flies to Peru for a weekend rendezvous with Robbie. Meanwhile, Steve, Barbara and Charley go on a camping trip and Barbara isn't exactly Mrs. Daniel Boone -- as Steve eventually finds out.
Barbara's natural enthusiasm bubbles over into an embarrassing evening for Steve and some of his business associates. Barbara explains to Steve that adrenalin gets her going in competition, and she is apprehensive when he is asked to play in his work's bowling team for fear she will make him lose concentration.
Katie Douglas takes a job singing in a small coffee house, but both Steve and Charley have their doubts about the man she is going to work for. After meeting the owner's wife, they realise she is in very good hands.
Polly fears that Chip is losing interest in her as a woman, and she takes some very dramatic steps to remedy the situation. Ernie gets the impression that the marriage is on the rocks when he finds out she is sneaking away from school and making mysterious calls to a man named Andre.
Uncle Charley is persuaded to fill in as a Cello teacher to a small group of reluctant young music students. His teaching methods leave much to be desired as he makes sure that they practice and practice for the upcoming recital.
Katie's triplets are spotted by a talent scout for a production company and are selected to appear in a television commercial. The temperemental Director yells when the boys misbehave and make a shambles of the studio. Grandfather Steve is asked to replace the actor doing the same so the boys will feel at ease.
Katie decides to put the triplets in a nursery school during the day when she fills in for a friend as a secretary, a move that enrages Uncle Charley when he hears about it. Later, it becomes apparent that he goes missing every afternoon causing the family to worry.
A little first grader named Alfred develops a big crush on third grader Dodie Douglas -- much to her displeasure. When Uncle Charley invites him to dinner, Dodie tries to fake illness to get out of it. Very soon Alfred's mother reports that he is missing.
When the Douglas men feel sorry for Katie because of her husband's continued absence, they proceed to overwhelm her with attention by each one of them taking her out to the same restaurant each night.
Dodie commits the entire Douglas family to a time consuming project - the awesome task of getting a four foot tall bag of Peanuts into small little sacks which they hope to sell at the school fair to raise money for Korean orphans.
Steve suddenly becomes accident-prone when his youngest son Ernie tries to discover by graph and observation how the moon's lunar phases affect people's behaviour.
While Steve becomes pre-occupied with a time-consuming project named Taurus, Barbara blossoms forth as a Championship Cook. Her recipe for 'Tangy Tidbits' wins her a trip to Hawaii for the final cookout.
Although Barbara is feeling very ill, she is saddled with the responsibility of looking after the entire Douglas household but in the end, to escape the pressure, she just walks out leaving Steve in a worried state.
The Douglas family becomes concerned about Dodie's despondency when Tramp, the ageing family mongrel, keeps disappearing at night. Meanwhile, Uncle Charley makes her a rag-doll and names it Arfie.
Katie becomes disillusioned about life without Robbie, and seriously thinks about divorce after speaking with another of the wives in the same situation. Meanwhile, Dodie and her two pals campaign to Barbara to let them have a slumber party.
Chip decides to quit his Chemical Engineering studies in favour of a rock music career when he meets up with a former friend who is now a big star. When Barbara visits John in the studio she is baffled by all the switches on the recording console, and realises that professional musicians know what they're doing, so how will Chip handle it?
Steve's boss Bob Anderson and his wife share 'Where-did-we-go-wrong?' panic as they struggle to communicate with their son; Steve and Barbara are the counselors. The teenager is rude and disrespectful to his parents but his insolence doesn't bother them nearly as much as not knowing if he is taking drugs.