Brunette Janet Wood, who works in a flower shop, and blonde Chrissy Snow, a guileless secretary, find Jack Tripper asleep in their bathtub full of water the morning after a going-away party for their ex-roommate Eleanor Garvey-Phillips.
However, first they must find a way to overcome objections from their landlord, Stanley Roper and his wife Helen, a romantically dysfunctional couple who live downstairs.
While the trio takes a break from weekend chores at the Regal Beagle; Jeff, a large chap, approaches their table to make a rude play for Chrissy.
After Jack and Janet find their apartment door unlocked and the cash missing, Chrissy arrives and insists that she left the envelope containing the rent money on a shelf.
A conflict over lack of privacy comes to a head after Janet brings home a date, and Jack and Chrissy have to cool their heels at the Regal Beagle.
Jack and Janet feel compelled to inform Chrissy that Lloyd Cross (Dick Sargent), a mature, sophisticated man she is dating, happens to be married.
Mrs. Roper reveals that she knows Lloyd's wife, which sends Jack and Janet rushing back to the apartment to break up Chrissy's romance.
To make matters worse, the mother goes into labor with her second baby that evening, and her husband faints, so both are hospitalized, stranding Jack and Chrissy overnight.
Jack and Chrissy take turns sucking up to Janet, who acquired a pair of tickets to a Frank Sinatra concert which she is not going to attend.
While repairing the sink, Mr. Roper inadvertently hears a conversation coming from the trio's bathroom, which includes Chrissy saying she "wants to get rid of it."
When Janet is terrified by a mouse in their bedroom, Jack takes advantage of the situation by offering to move in with Chrissy until the rodent is captured.
At the competition; Jack, out of honesty, reveals to Dean Travers about the incident, which leads to chaos that results in a free-for-all pie fight.
However, the supposed "other woman" is actually a real estate agent (Ruta Lee), who had been working with Mr. Roper to sell the building and find a new home in Cheviot Hills.
Fed up with the girls not doing their share and helping out at the apartment, Jack angrily leaves and becomes a live-in cook for Larry's boss (Jordan Charney) and his promiscuous wife (Cynthia Harris).
Ralph Furley, the new landlord, gets off on the wrong foot with the trio when he introduces himself to Janet and Chrissy at the Regal Beagle, who mistakenly think he's making a rude play on them.
Reverend Snow (Chrissy's father, Peter Mark Richman) has been given the opportunity to take over a church in Santa Monica.
The woman, Mrs. Claremont (Patricia Barry), is among the members of the selection committee who will decide whether to accept Reverend Snow into the church, but she does not approve of Chrissy's living arrangements.
A trouble-making 12-year-old boy named Billy, claiming to be homeless, wins the girls' sympathy and moves into the apartment, causing problems for Jack after he escapes from his foster home due to his foster parents making him do chores before going outside to play.Guest Stars: Shane Sinutko as Billy Todson and Joe George as Chauffeur.
Jack's first day behind the grill at a diner heats up hilariously when his attractive boss (Ellen Travolta) tries to spice up her life by making an unwelcome play on him.
Jack tries to back out of a ski trip with Larry and Janet until he meets a cute snow bunny named Inga (Laurette Spang).
A chance discovery of Larry and "Twinkie" together in front of a department store window (where Jack is currently employed as a mechanical mannequin) creates a brief love triangle between the three until it is revealed that "Twinkie/Bunny" has three other boyfriends on the side.
However, Chrissy's eager cousin Cindy Snow (Jenilee Harrison), using a somewhat clever process of elimination of the other would-be roomies, moves in after a rather clumsy introduction.
Realizing he got caught, Hadley shows up at the apartment, apologizes to Cindy, and begs her to take her job back with the promise of a raise and no more outside duties.
Jack plays cupid to his current girlfriend's widowed father (Keene Curtis), but his arrows misfire when the love-starved older man makes Janet his target.
A wealthy man, Winston Cromwell III (Jeffrey Tambor), persistently tries to convince Cindy to marry him, despite her repeated refusals.
Cindy breaks up with her boyfriend, a fashion photographer named Doug (John McCook), after he chronically criticizes her supposedly unfit facial features for modeling.
After an unsuccessful confrontation with Doug over this matter, Jack decides to make an honest woman of Cindy and proposes to her--before the whole situation is cleared up.
Later, when Jack is preparing food for Cindy's party, Terri tries to get on his good side by flrting with him, then apologizing when she is told by Mr. Furley that he is gay, but only makes things worse.
After nearly driving Terri to tears with hurtful pranks; Jack realizes he has made a horrible mistake, apologizes, and tells her she can move in.
Janet falls for a rich young man (special guest star Barry Williams) who invites her (and a date) to a stuffy, formal party on a small island.