how does act 1 create the main basis for the plot?

The exposition of the play is pretty clear. The Merchant of Venice opens with Antonio, a Christian merchant, in a depressed state. His friends try to cheer him up, but nothing works to make him feel better. Finally his friend Bassanio, an aristocrat who has lost all of his money, comes and asks Antonio to loan him some money.

Antonio, who has tied up all of his money is seafaring ventures, is unable to give Bassanio a direct loan. Instead he offers to use his good credit to get a loan for Bassanio. Bassanio finds Shylock, a Jewish moneylender, and convinces him to give a loan of three thousand ducats as long as Antonio will sign the contract.