The wealthy Buoso Donati has just died at home in Florence and his relatives are somewhat exaggerating in their mourning of his passing away; in reality they all expect to be amply consoled by their inheritance. But Betto di Signa, the poorest of them all, has heard strange rumours according to which Buoso has left all his possessions to the Friars Minor and the Charitable Works of Saint Reparata. Suspicious and worried, the relatives start looking for the will and, when they finally uncover it, they find their worst fears have come true. The false grief for the loss of their dear one turns into an authentic lament for the loss of their inheritance. Also the young Rinuccio is upset, for whom the money would have meant permission to marry a girl without a dowry, Lauretta, daughter of the commoner Gianni Schicchi.
In vain do Buoso's relatives turn to their elder, Simone, for advice on what to do; Rinuccio, however, suggests approaching Schicchi, well known for his shrewdness. The others reject this suggestion as the Donati family is above dealing with commoners, but by now Gianni, accompanied by his daughter Lauretta, has reached the house of the deceased, followed shortly after by the doctor, a certain Spinelloccio, still unaware of the death of his patient. As soon as Gianni sees how to turn the situation to his own personal advantage, he hides in the bed of the deceased, imitates his voice and sends the doctor away, claiming that he feels better and wants to rest. Gianni Schicchi's plan is now put into practice: dressed as Buoso, he lies on his bed and summons the public notary Amantio and two witnesses, in front of whom he dictates his new will. He distributes the cash and some properties fairly among the relatives, but the most valuable assets - the house in Florence, the mills in Signa and the mule - he keeps for himself, while the relatives can't do anything about it without revealing the fraud, for which the penalty is the cutting-off of a hand and exile from Florence. Once the notary and witnesses have left, Gianni chases the enraged relatives out of the house before they can plunder what has now become his property, while Rinuccio and Lauretta happily embrace.