Buoso Donati has died in bed. His relatives mourn melodramatically, until they hear the rumor that he has left all his money to the local monastery. They frantically search for the will. Rinuccio finds it, but refuses to release it to his aunt Zita until she agrees to his terms. If the will is favorable to them, she must allow him to marry Schicchi's daughter, Lauretta. Schicchi is looked down on by the Donati family since he is a relatively new arrival in Florence. Zita consents (she does not care whom Rinuccio marries so long as the will leaves them rich), and reads the will, as Rinuccio quietly sends for Schicchi. When the will confirms the rumor, everyone is furious. They refuse to allow Rinuccio to marry, and angrily turn down his suggestion that Schicchi, who is known for his clever schemes, can aid them.
Schicchi and Lauretta arrive to a cold reception. Schicchi, seeing how downcast the relatives are, uncharitably assumes that Donati must be better. He is informed otherwise, and attempts to console the relatives by mentioning their inheritances. Zita, touched to the quick by Schicchi's condolences, angrily explains the situation, and refuses to hear of a marriage. Rinuccio begs Schicchi to help. However, Schicchi, angered by his reception, refuses to help such people. He is persuaded to try by his daughter (O mio babbino caro), who is madly in love with the handsome Rinuccio. Schicchi reads the will, and proclaims that nothing can be done. But then, he has a thought, and Schicchi sends his daughter away so that she may be innocent of the knowledge of what he will suggest. Schicchi first orders the body to be moved to another room, and tells the women to make up the bed. He ensures that no one else knows of the death--but before he can explain, Donati's doctor arrives. The doctor is prevented from entering by the relatives, while Schicchi imitates Donati's voice, telling the doctor that Donati is feeling better. The doctor departs, praising his own skill. Schicchi explains, taking advantage of the darkened room and disguised as Buoso, he will impersonate Donati and dictate a new will.
Rinuccio goes to get the notary. The relatives mostly agree on the division of the property, except for Donati's mule (the best in Tuscany), some mills in Signa and the house they are in, the best assets. After a big fight they agree to let Schicchi decide who will inherit those items, but, one by one, they return to promise him a reward if he selects that person. Schicchi agrees to each bribe. Before the notary arrives Schicchi reminds all of the penalty for impersonation and forgery of a will: loss of a hand and permanent exile from Florence.
The notary arrives with the witnesses. Schicchi dictates a will that provides for a very modest funeral, a minuscule sum to the monastery and the agreed-upon division, as the relatives speak approvingly. But one by one, Schicchi grants the mule, mills, and house to himself, to the relatives' outrage. After the notary leaves, he throws everyone out and they are helpless to do anything except grab what they can on the way out the door. Now that Schicchi can give Lauretta a dowry, there is no obstacle to her marriage to Rinuccio. The lovers embrace, as Schicchi watches, moved. Schicchi turns to the audience and asks if this was not a fine use of Donati's money. He then requests the audience's indulgence, even if he did not receive Dante's, pleading extenuating circumstances.
-- courtesy of Wikipedia