Katie becomes a tutor to earn some extra money and is surprised to learn that her first student is not only a male, but a handsome ex-boyfriend of hers. Robbie doesn't mind at first, until he learns that the pair took a break from studying without his knowledge. A spat breaks out, and Robbie ends up sleeping on the couch until both of the newlyweds realize they were partly to blame.
Katie complains that Uncle Charley is making her feel unnecessary in the Douglas home. Steve speaks to Charley who tries to makes amends by planning a rather bizarre "tea party" on Katie's behalf. Meanwhile, Chip volunteers for the school dance committee, but the otherwise all-female committee frustrate Chip by ignoring his ideas. However, the girls want to help out after Chip tells them what Uncle Charley is planning for the tea party - party balloons and donuts for dunking are out, scones, silverware and napkins are in. Much to Katie's surprise, the tea party turns ...
Uncle Charley buys an 11-year-old trotting horse, thinking of the potential purses that the horse could win. With Charley himself driving the sulky, the horse wins his qualifying race, but he discovers his horse's opponents were run-down nags more qualified for the glue factory than competitive racing. So Charley retires the horse at a friend's stable, where Ernie can ride him whenever he wants.
Robbie reports to camp for two weeks of military reserve maneuvers, and Katie decides to go along and stay at a nearby motel so that the newlyweds can be near each other. Meanwhile, Uncle Charley and the boys are camping at Yellowstone, which also happens to be close to Robbie's camp. During a stroll, Katie wanders into the mock battlefield and gets arrested by Robbie's unit. Later, each of the other family members try to locate Robbie and similarly get detained. Steve heads toward camp to prevent the family from interfering with Rob's duties, but he manages to get ...
Uncle Charley is in charge of Frolics Night at his lodge, but has trouble lining up the entertainment, so Charley joins in with his lodge brothers, dressing in drag for a chorus line number. After the show, Charley falls asleep and embarrassingly has to walk home in costume after waking up. While resting on a park bench, an elderly man mistakes Charley for a woman and begins flirting with him, and Charley gets arrested for hitting the man.
Steve is on a business trip to Amsterdam, Robbie is in the South Pacific for military reserve training, and Chip is on a school trip to Mexico, so Ernie and Uncle Charley are left home alone together. Charley invites an old shipmate, Toby, over for dinner, not realizing he's now a famous TV star that Ernie idolizes. Steve is stuck in his hotel room working, Robbie can't leave camp to visit the local town, and Chip gets tonsillitis and thus misses out on seeing Mexico, meanwhile, Toby invites Ernie and Charley to the television studio where there are filming - ...
The Wong family from Bryant Park drops in on the Douglases unexpectedly while on their way to Hong Kong. Since the hour is late, Steve invites the Wongs to stay overnight, forcing him to cancel the family camping trip planned for early the next morning. The Wongs are too polite to tell Steve that they have family in Chinatown who are expecting them, and Steve make his family promise not to tell the Wongs that they ruined the Douglases weekend plans.
Rob and Katie invite a married couple - Denise and Larry - over to the house, but instead of playing "immature" games suggested by Rob and Katie, Denise, a psychology student, insists that each person take turns telling the others what he or she dislikes about them. Rob and Katie are uncomfortable with the idea, and Denise criticizes Katie for her comments about Rob. Eventually, Larry gets fed up with Denise's antics, jeopardizing their marriage.
When Steve is promoted to head of the helicopter division, the secretaries snoop into his personnel file and are happy to discover that their new boss is an available widower. Uncle Charley brings his helicopter loving girlfriend for a plant tour, but when denied entrance, he tells his girlfriend to pretend that she's "Mrs. Douglas" in order for her to get in. Meanwhile, Steve has invited Katie to visit the plant and tells her to let people know that she's "Mrs Robbie Douglas." When the nosy secretaries see Steve with two different women claiming to be "Mrs. Douglas,"...
Chip is miffed when a friend steals his girlfriend. Ernie's friend Shorty - who has a movie date set for Sunday - can't go due to a prior commitment, so the girlfriend innocently asks Ernie to walk with her to the movies, since he was going anyhow. But Shorty believes that Ernie is stealing his girlfriend and can't be convinced otherwise until Shorty helps Katie in with groceries and Robbie pretends that he thinks that Shorty is flirting with his wife.
Ernie's friends convince him to sell two worn out tires found in the Douglas' storage room to get money to buy more powerful squirt guns, but Uncle Charley notices the tires are missing and files a police report. A guilty-feeling Ernie eventually confesses to the family about his "crime," and Steve takes it a bit further and makes Ernie go down to the police station to admit to what he did.
The family is cleaning house together, with Katie doing a lot of heavy lifting and using strong chemicals. When Robbie finds her lying down after having fainted, Katie claims that she was just resting. Her mother arrives at the Douglas house unexpectedly, announcing that her woman's intuition tells her that Katie is pregnant, which a doctor visit confirms.
When it's time for Katie to go see a doctor for her pregnancy, a lot of people recommend their doctor. But her mother makes an appointment for her to see their family doctor Roy Osborne and Robbie is not exactly willing to go to him because he's a G.P. When they go to his office and see that he basically works out of his house Robbie is even more convinced that they should see a specialist. But Katie likes him.
After Katie feels her baby moving for the first time, expectant father Robbie decides it's time for him to mature and take life more seriously. Meanwhile, Ernie's friend's Gordon (Butch Patrick) has a dog that is also pregnant, with Tramp as the father, so Ernie also wants to be more involved with the birth of the puppies.
Steve meets with a hard-boiled Air Force general to sell a company product, but the general - himself a new grandfather - learns that Steve is about to become a grandfather, becomes an easy sell as he eagerly shares his experiences as a grandfather.
Rob and Katie realize that they will need help with the baby immediately after it is born, and seek out to hire a baby nurse. Uncle Charley wants the job, and is disappointed by not being asked, so he sets out on a series of baby sitting jobs to gain experience. He panics in his first experience with an actual baby and needs to call in Katie for advice, but he quickly becomes the favorite baby sitter in the neighborhood. While Rob and Katie realize he could do the job, they feel it is too much to ask now that Charley has a successful baby sitting career, but Charley ...
Katie feels like she is getting as big as a house, even bigger than her pregnant friend who is over due. While Robbie is away on military reserve training (again!), Steve takes Katie to her OB appointment where the doctor reveals why she is getting so large - Katie is having more than one baby! Katie doesn't want anyone to know before she can tell Robbie, but she's already getting her first labor pains and Robbie cannot be contacted.
With Robbie away on Army maneuver training, and still unaware that his wife is expecting multiple births, it is up to Steve and Uncle Charley to get Katie to the hospital when her labor pains start. At the hospital, rules only allow two people in the waiting room per expectant mother, so in turn Uncle Charley, Chip and Ernie each claim - unsuccessfully - to be the baby's father. Robbie finally arrives just as Katie is giving birth, and to his surprise, he is shown not one, not two, but three newborn sons.
Rob and Katie bring the triplets home from the hospital and reveal their names - Robbie Jr, Steve II and Charley - much to the delight of their namesakes. But Rob soon tires of all the attention that the triplets get and the repeated questions that everyone is asking - "How do you tell them apart?" But eventually Rob gets accustomed to the curiosity of his friends and neighbors.
Feeling that he's not contributing enough to the household expenses, Robbie decides to quit school, get a job, and move his family into their own place. Rob's college counselor urges him to think it over before rushing into such a major decision, and asks the office to delay his paperwork over the weekend. The stalling tactic gives Rob enough time to discover that despite the added financial burden placed on his father, Steve enjoys taking care of his grandchildren, and that he has plans to make an addition to the house to accommodate Rob, Katie's and the triplets.
It seems that everybody has something planned for Friday night - except Ernie. But since this will be the first time Rob and Katie are away from the triplets, they are reluctant to leave Ernie alone with the babies, so they hire a sitter recommended by the family doctor. However, the sitter (Rose Marie) seems to be more interested in watching television than watching the babies. After repeated calls from worried family members, she takes the phone off the hook, but now the constant busy signal causes more anxiety, so the family members all rush home, only to find that...
When Uncle Charley goes to pick up his date, Sally, he finds that not only has he been stood up, but Sally has run off to Las Vegas to get married. So Charley is in the dumps until a group of women fawn over him when he takes the triplets out to the park, and Charley tells the women that he is the grandfather.
The triplets are teething and Katie is exhausted from the constant care they require and from lack of sleep. She and her mother go to Santa Barbara for a few days of doctor prescribed rest and relaxation while the family takes care of the babies. When they return they find the house surprisingly spotless, only to discover that the family had hired three cleaning ladies in Katie's absence. Jon Walmsley (The Waltons) appears in a minor role.
Steve draws up plans to have one of the bedrooms enlarged to include a nursery. The contractor hired to do the work ends up locking horns with everyone except Katie, who manages to charm him. It turns out that his son's wife is expecting his first grandchild, and after never having a daughter, he's blessed with a granddaughter.