While on a research trip to Venice, the eccentric manga artist Rohan Kishibe finds himself inside a church confessional, not as a penitent but by chance, exploring the architecture. A masked Japanese man enters the opposite side of the booth, mistaking Rohan for a priest, and begins a harrowing confession. The man, Mizuo, recounts an incident from twenty-five years prior when he was a laborer in Venice. After refusing food to a starving vagrant named Sotoba, Mizuo caused the man to fall to his death. As the vagrant died, his ghost cursed Mizuo, vowing to inflict a despair greater than his own death at the peak of Mizuo's happiness.

Following the curse, Mizuo's life became inexplicably charmed; he accumulated immense wealth, married a supermodel, and had a beloved daughter, Maria. However, he lived in constant terror of the curse's trigger. The moment of supposed happiness finally arrives when he watches Maria playfully trying to catch popcorn in her mouth. At that instant, Sotoba's ghost possesses Maria and challenges Mizuo to a grotesque wager. To break the curse, he must toss a piece of popcorn high into the air and catch it in his mouth three times in a row; failure means death. After two successful catches, Mizuo fails the third, and Sotoba apparently decapitates him.

Fascinated by this bizarre tale, Rohan uses his stand ability, Heaven's Door, which allows him to turn people into books and read their life stories. He discovers that Mizuo is still alive because he had hired a look-alike butler to serve as a body double, who was killed in his place. However, this deception earned him a second curse from the butler's vengeful ghost, who now vows to claim his life at the peak of his daughter's happiness.

The story expands beyond the confessional as Rohan encounters the adult Maria, who is a mask-maker preparing for her wedding. Rohan realizes she is the same girl from the curse. Mizuo, desperate to survive, attempts to sabotage the wedding and even plots to murder the groom, Lorenzo, to prevent Maria's happiness from triggering his death. As Rohan investigates the labyrinthine canals and old palaces of Venice, the supernatural threat closes in, forcing Rohan to use his wits and Heaven's Door to confront the curse, which may ultimately be targeting him as well.