Harry, Sally and Tommy return to Earth, but are forced to bring along the Big Giant Head's niece (Roseanne Barr), who is assigned to act as Dick's wife in order to keep his focus on the mission and off Mary. To make matters worse, the police suspect that aliens might have landed in Rutherford and have sent Officer Don to investigate.
Dick comes up with a plan to get his new wife to leave him - by turning himself into the worst possible husband anyone could have. This doesn't work, and despite all his attempts to keep his wife secret, Mary soon finds out and breaks up with him, furious.
Still bitter after their break-up, Dick and Mary begin a childish prank war against each other to try to prove, ironically, who is the biggest adult in the office. Sally attends yoga classes with Mary, where she begins to take a liking to her instructor (Leigh McCloskey). At home, Tommy and his friends have formed a garage band and are practicing in the garage, while Harry wants desperately to be their roadie.
Mary is unable to tell her bitter, condescending parents (Elaine Stritch and George Grizzard) that another relationship has fallen through, and so invites Dick to meet her parents under the pretense that they are still to be married. Dick is quick to accompany her, thinking that if he can win her parent's hearts he might win back Mary's. Back in Rutherford, Sally, Harry and Tommy help Nina to move out of her apartment, before Sally decides that she'd like to move in.
Dick goes for a medical checkup but becomes scared and leaves before his appointment. Mary asks Sally and Tommy to house sit for her on Halloween, to stop trick-or-treaters (Emile Hirsch) egging her home while she is out. Harry, however, chooses to stay at home, where he encounters some very strange noises and starts to believe that there is a ghost in the house.
When Dick discovers that he has put on a large amount of weight, Nina tells him that it is because he is sad about his breakup with Mary. Dick tries to slim by eating healthily and attending exercise classes, but nothing works and he ends up joining a weight loss group, where he runs into Mary. At home, Sally begins to grow attached to her new tomato plant and names it Jeremy, leading Don to fear the worst when he hears that Sally is spending lots of time with 'him'. Meanwhile, Tommy is worried that August is cheating on him, and sends Harry to investigate.
Dick gets jury duty for the first time and struggles to deliver a guilty verdict for a clearly guilty man. Meanwhile, Sally, much to Officer Don's distress, falls for a cockney 'artist' called Seth, and makes herself over to look and sound just like him. Elsewhere, Harry and Tommy volunteer themselves for Mary's class project - they must communicate without words for five days.
Dick has two theater tickets for Peter Connolly's 'King of the Jig' left over from when he and Mary were dating, but upon realizing that he doesn't have any friends to accompany him, he forges a friendship with Don and becomes jealous when Don wants to spend time with his other friends. Meanwhile, Sally's 'artist' boyfriend Seth voices his dislike of Connolly and Sally decides to lure him to Johnny Foam's, while Tommy forces Harry to go to night school and earn a diploma, so that he can have the things his friends do.
Tommy decides that he needs to find a woman who is closer to his real age, and pursues Mary, kicking up a fierce rivalry between himself and Dick. Elsewhere, Sally finally gets a proper job, but her boss begins to sexually harass her, while Harry is under orders not to watch any TV for a week, since Dick thinks he is watching too much of it.
Dick takes care of a felon called Eddie because he feels he can make an honest man out of him, however things just end up going wrong. Dr. Albright is fed up with her dog Pepper and enlists Sally's help in finding a new home for him. Back at home, Harry struggles to complete a book report on Little Women for night school.
Mary's father arrives in town, announcing that he and his wife are separated, but unfortunately for Mary, he soon begins dating Sally. Meanwhile, Harry and Tommy stumble upon Pendleton's old radio station, where Harry takes the controls and begins a new-found career as a shock jock.
Dick demands that Sally and Harry get jobs, so Harry gets work in a bar (Happy Doug's) and Sally as Mary's research assistant. However, upon finding out that Sally has chosen Mary's subject (anthropology), he worries that physics is too boring and strives to spice it up for Tommy's career day. After this goes badly wrong he decides that he wants to be a cop, like Don.
After Mary and August tell the Solomons that they can be a little weird sometimes, they strive to become a completely average family, going as far as to move to a new, average, apartment, make themselves over with new, average clothes and behave in the most average way they can. Unfortunately for Dick, Mary is soon turned off, missing his old weirdness.
An eccentric new professor (John Cleese) has arrived at Pendleton University and Dick is quick to befriend him; he soon becomes annoyed when Dr. Neesam appears to be able to do everything better than he can, and becomes a rival for Mary's affections. Sally becomes concerned that Officer Don has lost interest in her after hearing a woman tell him he was "great last night", so Mrs. Dubcek convinces her to disguise herself and follow him around.