It's 1979 in New York City, Marvin, his son Jason, his psychiatrist Mendel, and his male lover Whizzer are Four Jews In A Room Bitching (well, technically, Whizzer's only "half Jewish"). Marvin steps forward to explain his situation: He has left his wife, Trina, for Whizzer, but Marvin wants A Tight-Knit Family and is attempting to forge a new family situation with the addition of Whizzer, a situation no one is that happy with.
Trina, on Marvin’s recommendation, pays a visit to Mendel where she wearily wonders how her life has turned out this way. Mendel, who is instantly attracted to her, tries to console her, telling her that Love is Blind. Meanwhile, Marvin and Whizzer comment on their relationship: the two have very little in common, apart from the fact that they both love fighting and are insanely attracted to each other. Both worry that The Thrill of First Love is wearing off.
The cast presents an interlude: Marvin at the Psychiatrist, a Three-Part Mini-Opera. In part one, Mendel asks Marvin about his relationship with Whizzer and Marvin weighs the pros and cons of the relationship, ultimately concluding that he does love Whizzer. In part two, Mendel shifts the topic to Trina, and the session becomes one where Mendel, obviously aroused, interrogates Marvin about his ex-wife's bedroom habits. In part three, Marvin and Jason provide counterpoint on their strained relationship.
Jason, who is 10, is very worried that because, as he puts it, "My Father's a Homo", that he'll turn out to be one too, and is very afraid of turning out like his father. Because he is worried, he acts up, and Everyone tells Jason to See a Psychiatrist immediately. It is only after Whizzer softly adds his voice to that of his parents that Jason agrees to see Mendel.
It is very clear that Marvin is trying to pigeon-hole Whizzer into the role of homemaker, and they fight. Meanwhile, Trina complains to Mendel how her role in the family dynamic is being phased out as Whizzer becomes increasingly prominent in Marvin and Jason's lives as Marvin continues to insist that all participants get along together as one extended family. All agree that "This Had Better Come To A Stop".
Jason is acting up again, and Trina phones Mendel frantically to "Please Come To Our House" for dinner and therapy. Mendel arrives and immediately charms Trina. He and Jason settle down for Jason's Therapy, in which Jason frets about his future and Mendel, in a very round-about way, encourages him to simply relax and enjoy life. After several such dinner/sessions, Jason asks Mendel what his intentions are towards Trina, and Mendel makes A Marriage Proposal. Clumsy and neurotic though he may be, he's sincere and Trina accepts him, to Marvin's fury. He is losing his Tight-Knit Family (Reprise), and also his therapist.
In Trina's Song, the titular singer reflects on her situation. She is tired of the man's world she lives in, and even though she knows that Mendel is the same kind of man Marvin is, slightly childish and neurotic, he loves her and she could do a lot worse. She may not be exactly happy, but he's hers. In contrast, the four men sing a hymn to masculinity in all its aspects, the three adults singing in a falsetto to match Jason's unbroken voice, in the March of the Falsettos.
Marvin teaches Whizzer to play chess, but bitterness and ill-feeling boil over The Chess Game until the fight to end all fights breaks out between the two and they break up. Meanwhile, Trina and Mendel move in together and start Making a Home. As he packs, Whizzer reflects on "The Games I Play" with his own heart, and finally comes to the conclusion that he does not love Marvin.
Trina and Mendel send out wedding invitations, and Marvin goes crazy. He confronts Trina and incoherently accuses her of trying to ruin his life, finally breaking down in rage and slapping her. Shocked by his actions, both reflect that "I Never Wanted To Love You", a sentiment Whizzer repeats to Marvin and Marvin repeats to Jason and Whizzer.
Marvin is finished with Whizzer, and his relationship with Trina is in tatters, but Marvin can at least salvage his relationship with Jason, who has just discovered women to his immense relief. Marvin sits down Jason for a talk Father to Son and tells him that he loves him, and no matter what kind of man Jason turns out to be, Marvin will always be there for him.