How does Antonio and Bassanio's friendship set the whole basis of play?

Antonio and Bassanio's friendship gets Antonio into serious trouble with Shylock, but it also rescues him from that trouble.
The main plot of the play has to do with the "bond" (legal contract) Antonio signs with Shylock: if Antonio does not pay his debt on time, Shylock is entitled to a pound of his flesh. Antonio does not take out the loan from Shylock for himself. He does it so he can give a loan to his friend Bassanio.
Antonio has lent Bassanio money in the past, and Bassanio for various reasons has always been unable to pay him back. For most friendships, this would cause the person in Antonio's position to end the friendship, or at least stop lending. Many people in Bassanio's position would hesitate to ask for yet another loan from Antonio. These two are very confident in each other's friendship, though. Bassanio asks for a loan so he can woo Portia, and Antonio gives it and even says not to worry about the past defaults.
At the time of his bond with Shylock, Antonio does not dream that he'll be unable to pay; he is very rich and has many ships out on moneymaking ventures. Only when every single one of his ships are wrecked is he in danger of having to pay a pound of flesh. 
By this time, Bassanio has married Portia, who is an heiress. Bassanio now has the money, and he is willing to pay Shylock back the amount of the debt. When Shylock insists on having Antonio's flesh instead, Bassanio offers to pay up to ten times what Shylock lent, but Shylock will not accept it.
Bassanio is unable to save Antonio financially. He does, however, save him indirectly, by being married to Portia. When Portia learns of the trouble her husband's friend is in, she sneaks off to Venice, dresses up as a young lawyer, and saves Antonio in a court of law by proving the bond does not allow Shylock to draw any blood.  
Bassanio is unaware until afterward that the young lawyer is Portia, so he certainly is not in on the plan. Nevertheless, it is the fact of his being Antonio's friend that brings Portia to Antonio's aid. She wants to help her husband's best friend. In fact, she insists he be helped at once.
If Bassanio had not married Portia, she would not have heard about Antonio's plight. On the other hand, it was in order to marry Portia that Bassanio caused Antonio to take the fateful loan from Shylock. So it is the friendship between the two men, as well as the love between Bassanio and Portia, that cause the trouble and later solve it. These relationships drive a large part of the plot.