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 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.

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.

Barbara is flustered when she receives a phone call from a former college suitor, and is very apprehensive when Steve invites him to dinner. The entire family is anxious to meet her old flame, thus compounding her uneasiness.

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.

While the family is doing some spring cleaning, Katie faints behind the couch. Soon her mother's intuition brings her to the house and she announces that Katie is going to be a mother. Once it is confirmed by a doctor, the family dotes on Katie, but seems to ignore the father to be.

Although Katie confidently calls the look-alike triplets by name, other members of the family doubt her accuracy, and when Steve helpfully bathes them, she admits that she can no longer tell them apart because he washed off the inkdots put on their feet. They must be taken back to the hospital for proper identification.

Chip asks Steve's permission to elope, but before he grants it, he insists Chip give Mr. Williams the courtesy of asking his approval too. Chip goes to see Mr. Williams only to have him accusingly flaunt a private detective's report. Realizing that Polly's own father had hired a man to trail them, he storms out before asking for her hand.

An old high school sweetheart calls for Steve while he is out. Feeling nostalgic, Steve tries to locate her in town, but never seems to be able to catch up with her as he reminisces about their past relationship. As he arrives home the door bell rings and he gets a disappointment then a surprise.

Steve takes a week's vacation from the family where he soon finds that he can fall in love with someone as easily as fall out of love with them. Sure that an older couple he's met are trying to play matchmaker, Fran is unsociable towards Steve at first until he points out that he only came along on the trip to appease his fellow campers.

Steve is going to Chicago on business and he's promised to take Chip with him. A change of plans sees Steve boarding a military plane bound for Paris. Chip takes action when he finds out that he can't go on a trip to Paris with his father, by sneaking onto the plane. He gets lost in the city and is befriended by a little girl named Marie.

Chip puts his father into the uncomfortable position of defending the importance of his profession. Chip is unenlightened by the tour of the Air Force base until a full-scale emergency arises and Steve is asked to help a desperate pilot down from the air.

In an advanced stage of pregnancy, Katie suffers qualms about her looks that are as out of proportion as her figure. She is temporarily distressed by her hugeness until she learns from her doctor that she is going to have more than one baby.

Dressed as a cat for a costume party at the home of his employer in Bel Air, Steve goes for gasoline when his car stalls but the police arrest him as a burglar. Meanwhile, Polly notes that there is so much love in the Douglas household, that she just has to become a part of it.

After Chip and Polly return from their honeymoon they spend a few days at the Douglases. Soon they move into their own college dormitory, and bridegroom Chip faces a father in law who refuses to speak to his newly married daughter.

Level-headed Katie becomes a jealous, suspicious wife when a pile of circumstantial evidence stacks up against Robbie, as she suspects him of seeing another woman.

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?

Mike wants to go skiing on the weekend with his friends but must also keep up his Spanish studies. So he adopts the learn while you sleep method via use of a pre-programmed record player. Steve's room has just been painted so he decides to sleep in Mike's bed. Mysteriously, he wakes up speaking Spanish, and the next night Bub does the same. Before long they are both uttering phrases they couldn't possibly know.

Robbie is heading for an ‘F' in world literature until the teacher assigns a beautiful blonde newcomer as his study partner. He is attracted to her right away but soon discovers that her looks don't even compensate for her loss of mind.

Steve and Bub are both called out of town and Mike urges them to leave him in charge, only to find the role of mother hen harder than it looks. His worth is really put to the test when he learns that Robbie and Hank have been taken to hospital after an accident at school.

Two dates, confused phone calls and name mix-ups create a tangled evening for brothers Mike and Robbie, when each get's the other's date. Mike gets a shock when he picks up a fourteen year old, while Robbie nearly croaks at the sight of his 'older woman'.

Steve's friend asks him to come up with a solution when she thinks her daughter is dating an older man. Steve is shocked when he sees that the initials of the fraternity pin the man gave her identifies his oldest son Mike. However it is really Robbie who took the use of Mike's pin to fix his button-less shirt.

It may not be poker, but nine in the Douglas family is a full house. Feeling somewhat responsible, Robbie decides to quit school in order to pursue work full time and find a new home for his family. A wise school counsellor takes steps to discourage the move. Meanwhile Steve designs some plans for an extension to the house to accomodate the babies further.

Steve's old Chinese friend transfers to Los Angeles and happily renews his friendship with the Douglases. He confides in Steve and Charley that he is concerned about the man his daughter has married - an aparently aimless hippie with long hair.

Uncle Charley and Barbara suspect that Chip and Polly are becoming serious about one another. Polly, unhappy at home because of her father's strictness, is afraid of losing Chip because of her parents' meddling. She asks Chip to elope, and taken aback, he asks his father for advice.