Archy Eddie Bracken, a New York City poet, attempts suicide and returns as a cockroach. He discovers he can write poetry by tapping his feet on a typewriter, and as he adapts to his new life he grows attached to Mehitabel Carol Channing, a bright and singing alley cat. She, however, fancies the handsome tomcat Big Bill Alan Reed, who is not quite ready to settle down. When Big Bill dumps Mehitabel, Archy confronts her about her wild ways and her attraction to bad-boy tomcats in general, a topic that unsettles her as she tests her own boundaries.
Tyrone T. Tattersall John Carradine, a self-appointed theatre maestro cat, promises to make Mehitabel a star and becomes her latest lover. Archy attempts another suicide, though this time it fails to shake him from his resolve to understand love, art, and loyalty. In the theatre, Mehitabel fulfills her end of the arrangement by bringing food for Tyrone, yet he cruelly kicks her off the stage. Archy and Big Bill watch from the wings as Mehitabel drifts back toward Big Bill, rekindling their old dynamic and leaving Archy to grapple with unrequited affection.
Back at his typewriter, Archy rediscovers his urge to rally the insects and spiders for a revolution, but he drops the idea the moment he learns that Mehitabel has kittens and that Big Bill has left the scene again. On a rainy evening, Archy points out to Mehitabel that her kittens—inside a coverless trashcan—are drifting away, and together they rescue them. Yet Mehitabel, now moody and caught between worlds, chases Archy away, arguing that she must attend to her own private business.
Archy presses on with a plan to change their lives: he persuades Mehitabel to abandon the alley lifestyle and take a job as a house cat to support the kittens in an upscale home. Later, when Archy visits again, Mehitabel appears altered by the experience. She reminds him that social class now separates them from friendship, and she tells him goodbye—only to regret the moment later. Archy sinks into a melancholy, drinking and observing a cast of colorful street figures, including ladybug street walkers who find charm in his poetry about Mehitabel, while Big Bill often mocks him from the sidelines.
Mehitabel eventually returns to Shinbone Alley and once more sings and dances with a vitality that echoes her old spirit, suggesting that reforming her may not be the right path after all. After a final volley of sentiment and irony, Archy comes to a quiet, affectionate acceptance of Mehitabel as she is, choosing to celebrate her wild essence rather than try to change it. In the end, Mehitabel and Big Bill continue their playful chase, a testament to a partnership that survives through difference and a shared history of poetry, performance, and street-lit love.
