This is the scene where Shylock is to take his forfeiture from Antonio. Antonio's friends and even the Duke beg him to have mercy, Shylock says he will not grant mercy for the simple reason that he hates Antonio. He says Christians do what they wish with their slaves because they have bought them, and so it is with Antonio. Antonio wishes his friends to beg no more, he is resigned to his fate. Nerissa then arrives disguised as a lawyer's clerk with a letter from Bellario, the lawyer who was supposed to oversee the case. The letter says Bellario is ill and has sent a young man of good intellect to stand in his place. This young man is Portia in disguise as Balthazar.
She, too, asks Shylock for his mercy but he stands firm. She then says that by the laws of Venice he is entitled to his pound of flesh, and Shylock praises the decision. She stops him short, and explains further that the bond entitles him to a pound of flesh, but no blood. Shylock is stunned, so he asks simply for the money and he will forego the pound of flesh. Portia pushes on, saying he has already denied the payment and so now must take the flesh. She warns him that if he spills a drop of blood he will die. He therefore refuses to take the flesh and wishes to leave, but Portia catches him again. She cites a law in Venice stating that if any alien (as Jews were) tries to take the life of a citizen, half their wealth goes to that citizen and the other half goes to the state. Furthermore, his life is in the hands of the Duke. The Duke spares his life to show him what "Christian mercy" is. Shylock asks that his life be taken because it is already over without his means of life. Antonio adds further to the deal stating that his half of the money be left with Shylock with the promise that he give it to Lorenzo in his will and that he also become a Christian. Thus, Shylock is forced to accept the marriage of his daughter to a Christian, turn Christian against his will, and lose half his wealth. This is what they call Christian mercy.
Portia, still disguised as Balthazar prepares to leave, but Bassanio stops her, insisting that he be allowed to give some token of thanks for her saving Antonio. Portia contrives to ask Bassanio for the ring he gave her, and test his loyalty to her. He refuses her at first, and she leaves with a speech about his ungratefulness. Bassanio changes his mind and sends Gratiano to catch up with her and present the ring.