Act I
Scene 1: (Venice) The merchant Antonio has made risky investments, but on learning of his friend Bassanio's need of money in order to go to woo Portia, Antonio offers to be his guarantor if he can borrow money.
Scene 2: (Belmont) Portia outlines her situation, the method of choice of a husband by casket fixed by her dead father, on condition that if suitors choose wrongly they promise never to marry at all. Present suitors shown as comic buffoons.
Scene 3: (Venice) Shylock's past treatment revealed in his discussions with Bassanio; Antonio agrees to the forfeit clause of a pound of flesh.

Act II
Scene 1: (Belmont) The exotic Prince of Morocco arrives.
Scene 2: (Venice) Young Gobbo (clown) meets his old father; Gobbo becomes Bassanio's servant, instead of being Shylock's.
Scene 3: Jessica (Shylock's daughter) involves Gobbo in an elopement plot, she wishes to marry Lorenzo.
Scene 4: Lorenzo elaborates the elopement plan.
Scene 5: Shylock leaves Jessica guarding the house.
Scene 6: The elopement of Jessica and Lorenzo.
Scene 7: (Belmont) Morocco chooses the golden casket after long description and analysis; "all that glisters is not gold."
Scene 8: (Venice) Shylock's reactions to the elopement reported; Antonio takes leave of Bassanio.
Scene 9: (Belmont) Prince of Arragon chooses silver casket. Bassanio's arrival announced, Portia (who has seen him in Venice) hopeful.

Act III
Scene 1: (Venice) Reports of Antonio's losses arrive; Shylock hears how Jessica is spending his money wildly.
Scene 2: (Belmont) Bassanio chooses the leaden casket and wins Portia; poetry of intense love interrupted by news of Antonio's ruin; servant Gratiano makes parallel marriage with Portia's servant Nerissa; Portia sends Bassanio with money to help Antonio, giving him a ring as a sign of their enduring love, never to leave his finger; Nerissa likewise with Gratiano.
Scene 3: (Venice) Antonio is arrested.
Scene 4: (Belmont) Jessica and Lorenzo arrive at Portia's house and are left in charge while Portia goes to pray (in fact to go to Venice with Nerissa).
Scene 5: (Belmont) Comic dialogues between Jessica, Lorenzo, Gobbo.

Act IV
Scene 1: (Venice) The Trial Scene: Shylock rejects offers; Bellario (Portia disguised as a lawyer) arrives; makes speech on Mercy; Shylock demands the Law; Bellario gives Shylock the right to cut, then at the last moment reminds him that he must shed no drop of Antonio's blood (the bond only spoke of flesh); in punishment, Shylock looses all, and must become a christian; Bellario (Portia) asks for Bassanio's ring as a reward; he refuses but Antonio forces him to send it.
Scene 2: Portia/Bellario receives the ring, Nerissa decides to attempt the same with Gratiano.

Act V
Scene 1: (Belmont) Rhapsodic duet between Lorenzo and Jessica. The returns are announced; there is harmony and music. Portia returns first, then Bassanio etc arrive; Nerissa notices Gratiano's ring is gone, he explains, Portia criticizes him, until he tells her that Bassanio has done the same: "I will never come to your bed until I see the ring." Portia rejects all his arguments, saying that the lawyer now has the title to her body; Antonio intercedes, Bassanio repents; Portia produces the ring, "the lawyer was in my bed last night!" Rapid explanations follow, and reconciliation, after which Portia reports that Antonio's lost ships have come in safely; in future, all agree, they will care for their wives' ring!

A very ancient and widespread pound of flesh story seems to have been the source, in the form of a late 14th century Italian novella by Ser Giovanni, Il Pecorone, which Shakespeare perhaps read in Italian. Here we find all the main elements of the plot except for the choice of a husband by means of a choice of caskets, which is also a traditional narrative motif. When the play was published in 1600, from Shakespeare's manuscript probably, the title page mentions 'the extreame crueltie of Shylocke the Jewe towards the said Merchant, in cutting a just pound of his flesh' and this has always been more Shylock's play than Antonio's.

Until the 19th century, Shylock was usually acted as a comic villain; certainly there is some influence from Marlowe's Barabas in him. As the modern theater developed, however, Shylock was acted with more complexity, demanding more sympathy for his situation. In post-war European productions, the theme of anti-semitism has made the whole play something of a problem. Shylock as a Jew is loaded with extra layers of reference to recent history, a burden that the play's fragile thematic unity cannot really bear.

The great diversity of themes and plots is typical of Shakespeare's 'romantic' comedies, although the suspense during the trial, when it seems that Antonio must die in a messy way, introduces the more serious note of the question 'What price will love pay? What is love worth?' This is one main theme of the play. The speech on Mercy that Portia, disguised as a lawyer, makes early in the Trial Scene has become famous in its own right:

The quality of mercy is not strain'd,
It droppeth as the gentle rain from heaven
Upon the place beneath: it is twice blest,
It blesseth him that gives and him that takes,
'Tis mightiest in the mightiest, it becomes
The throned monarch better than his crown. (Act IV.i)