"MM!" is a romantic comedy anime centered around Sado Tarou, a high school student with a peculiar psychological condition: he is a masochist who derives pleasure from being mistreated by others. Desperate to overcome this trait, Tarou joins the Second Volunteer Club, a school group dedicated to helping students with their problems. The club is run by Mio Isurugi, a seemingly sweet and kind girl who harbors a deeply sadistic side, making her the perfect foil for Tarou's masochism. Their dynamic forms the core of the series, as Mio frequently subjects Tarou to humiliating and painful situations under the guise of "helping" him.

The narrative explores Tarou's attempts to navigate his condition while dealing with the eccentric members of the Second Volunteer Club. Alongside Mio, the club includes Arashiko Yuuno, a shy and reserved girl with a hidden fetish for cross-dressing, and Tatsukichi Hayama, a childhood friend of Tarou who often finds himself caught in the chaos. The series delves into their interactions, blending humor with moments of introspection as Tarou grapples with his identity and relationships.

Throughout the story, various subplots emerge, including Tarou's interactions with other students and his efforts to understand his feelings for Mio, who remains both his tormentor and confidante. The series balances slapstick comedy with lighthearted drama, often highlighting the absurdity of Tarou's situation while touching on themes of self-acceptance and the complexities of human relationships. The characters' quirks and the exaggerated nature of their personalities drive much of the humor, creating a series that is both entertaining and unconventional in its approach to romantic comedy.