The Merchant of Venice is a complex play with several subplots. However, the central story might be summarized as follows. Antonio is a wealthy member of Venice's ruling class of merchant princes. His dearest friend, Bassanio, a member of the same social class who lacks Antonio's wealth, asks Antonio for a loan of 3,000 ducats, which he needs for the incidental expenses involved in wooing the wealthy and beautiful Portia. Antonio, who does not have the money on hand, borrows it from Shylock, a member of the societally despised Jewish moneylending class. Shylock inserts a condition into the bond for the loan, saying that if Antonio fails to repay him, he will be entitled to cut a pound of flesh from the Antonio's body.

Bassanio goes to visit Portia, passes the test set by her late father for prospective suitors, and wins her love. The two marry. Meanwhile, the bond for Antonio's loan has fallen due, and Antonio has been unable to pay. Bassanio goes to his aid with some of Portia's money to repay Shylock, but it is too late. There is a trial before the Duke of Venice in which Shylock demands his pound of flesh. However, a young lawyer named Balthazar, who is really Portia in disguise, comes to defend Antonio and points out that the bond entitles Shylock to flesh, but not blood.

Since there is no way to cut flesh without shedding blood, Shylock is defeated, and the Duke imposes heavy financial penalties upon him, as well as forcing him to convert to Christianity. The play ends with Portia revealing her identity to Bassanio and the revelation that Antonio's fortunes have been restored, as ships he had thought lost have now come safely to port with their cargoes.

Bassanio needs money to woo the wealthy Portia. He asks his friend Antonio, a wealthy merchant, for a loan. Antonio's money is tied up in merchant ships carrying back goods from different parts of the globe. Antonio has no money to spare, but because he loves Bassanio very deeply he takes a loan for his friend, from Shylock.

Shylock is a Jew. They were the moneylenders in Venice, as Christians were not able to charge interest on loans. Antonio had insulted Shylock in the past for being Jewish, so Shylock wanted revenge. He loans Antonio the money, but he insists on a pound of Antonio's flesh, taken from any part of his body Shylock chooses, if the money is not repaid on time.

As is declared in her father's will, Portia is not able to choose her husband. Instead, she must marry the man who picks the right one of three caskets, one gold, one silver and one lead. Several suitors select the wrong casket. Bassanio chooses the right one, made of lead. He also falls in love with Portia, although he initially wished to marry her for her money.

Meanwhile, all of Antonio's ships appear to be lost. Shylock  then insists on his pound of flesh. He wants it from Antonio's heart, which means Antonio will die. The two go to court to plead before the Duke. There, Portia and her maid Nerissa show up, disguised as a male lawyer and his male clerk. Portia proceeds to appeal to Shylock's sense of mercy, and when that doesn't work, reverts to the letter of the law, as Shylock has done. She insists that Shylock can take Antonio's flesh but must not spill one drop of his blood or his own life will be forfeit, a sentence the Duke supports. Shylock is humiliated and defeated, and is sentenced to  give up half his fortune, and to convert to Christianity.

After the trial, the disguised Portia and Nerissa insist that their fiances give them rings that they had promised never, ever to part with under any circumstances. They had given them, however, to the "male lawyer and clerk" as repayment for the positive outcome of the trial. Later, they tease their beloveds about this, but all works out harmoniously in the end, especially as three of Antonio's ships arrive at port after all.

A fortuneless nobleman, Bassanio, asks for a loan from the merchant Antonio so that he can pursue Portia, a wealthy heiress, in order to restore his fortune. Since the merchant has all his assets tied up with ships that at sea, he tells Bassanio he will use his credit to obtain a loan from the Jew Shylock.  Because Shylock fiercely hates Antonio, he latches on to the opportunity to get revenge; he agrees to loan Antonio the money. But, if Antonio does not repay this loan, Shylock will get a pound of flesh as payment from him.

With the money he needs, Bassanio pursues Portia, and he selects the casket (small box) that allows him to marry her. But, in the meantime, Shylock's daughter runs off with a Christian, converts, and steals her late mothers jewels. The avaricious and offended Shylock is enraged by this; he vows to exact the flesh from Antonio, who is unable to pay the loan because his ships are lost at sea. When Portia learns from Bassanio of Antonio's plight, she disguises herself as a doctor of law and, with her servant Nerissa dressed as a law clerk, Portia appears at court on Antonio's behalf where Shylock demands of the Duke, 

I have possessed your grace of what I purpose
And by our holy Sabbath have I sworn
To have the due and forfeit of my bond" (4.1.34-36).

Portia's speech before the court is clearly a statement of theme for this drama:

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. (4.1.186-192)

While it seems that Portia goes along with the sentencing, when Shylock takes steps to cut off his pound of flesh, Portia/the doctor of law points out that there was no provision in the conditions for Antonio's blood, so Shylock can only have flesh--no blood. Defeated, Shylock is further humiliated as he is made to convert to Christianity. Antonio's ships finally arrive and the lovers, Bassanio and Portia, are married.