Michele is the owner of a barge on the river Seine. He has a much younger wife, Giorgetta, whose feelings towards him have cooled since the death of their child. She has secretly begun a passionate affair with Luigi, one of Michele’s workers. He and Giorgetta dream of living together in the Paris suburb of Belleville, where Luigi comes from. The “rummager”, the wife of the stevedore named "mole", who ekes out an existence collecting old clothes, also has a vision of a peaceful life in the country.
Luigi can no longer endure the fact that Giorgetta is another man’s wife, and wants to leave the barge in Rouen, but Michele persuades him to stay. Luigi and Giorgetta agree on their next rendezvous – she will light a match as a sign that he can come on board.
Michele reminds Giorgetta of the happy times they have had together, but she rejects him. Left alone, Michele wonders who his wife’s lover could be, and resolves to kill him. Before going to bed he lights his pipe on deck, and Luigi, mistaking this as the agreed sign, runs into Michele’s arms. Michele kills him and hides him under his cloak. Giorgetta comes on deck, longing to see her lover. She asks Michele to warm her. He opens his cloak and the body falls towards Giorgetta.
The rich Buoso Donati has just died, and the relatives who have hastened to the deathbed expecting a big inheritance, are weeping false tears for the deceased. A rumour starts circulating that Buoso has left his entire fortune to a monastery. A feverish search is immediately started for the will. Finally Rinuccio finds it. He sends for Gianni Schicchi and his daughter Lauretta, whom he loves and – against the wishes of his family – wants to marry.
Buoso has indeed disinherited his relatives. The assembled persons are unwilling to ask the advice of Schicchi, a newcomer. Neither does he want to have anything to do with the arrogant Florentines, but Lauretta sings him a heartrending plea to make her marriage to Rinuccio possible.
As no one has yet learned of the death of Buoso, Schicchi takes on the role of the deceased, disguising his voice to deceive the doctor when he comes to call. To everyone’s delight, Schicchi decides to draw up a new will; but before this he reminds them of the dreadful penalties that threaten those who forge wills: their right hands are cut off and they are sent into perpetual banishment.
The notary having been summoned, Schicchi now dictates his last will, disguised as Buoso. As previously agreed, the money and the estates are distributed between the relatives. But the main part – the mills at Signa, the Florentine town house and the valuable mule – are bequeathed by Buoso to his "faithful friend Gianni Schicchi". The new house owner chases the cheated relatives out.
Lauretta and Rinuccio are now united forever. Gianni Schicchi asks the audience to forgive his roguish trick, pleading "mitigating circumstances".