Everything is in a relatively uncomfortable place to begin with. In Venice, Antonio is sad, likely because he knows he's losing Bassanio to Portia. Portia, meanwhile, is not in her natural state either. Her father has died, and her biggest problem is this awful lottery he's set up, rendering her unable to choose her own husband.
Everyone has at least one conflict here. Antonio knows he's losing Bassanio, but he'll still do anything in his power to make Bassanio happy. He asks for money from Shylock, but Shylock explains the twisted past the two have together, full of hatred and mutual religious animosity. Shylock is willing to lend Bassanio the money on the weird condition that Antonio sign away a pound of his flesh. Bassanio senses this is a bad idea, but Antonio is pretty sure it will all work out. As he's already confessed he is meant to play a sad part, we're sure something will go awry here. Bassanio, at Portia's house, is faced with the fact that he must pick the right casket or never see her again. Tortured by this uncertainty, Bassanio insists on playing the casket game as soon as possible.
No sooner than Bassanio wins Portia, a letter arrives announcing Antonio's sad fate: his ventures have failed and Shylock wants his flesh. Shylock seems to be more enraged than ever, and the fact that Jessica ran away with his money adds to the anger he seems to take out on Antonio. Shylock seems perversely committed to having Antonio's hide, not the money, and Antonio is resigned to his fate. He asks only that Bassanio come and see him before he dies (which complicates Bassanio's good luck – he has to immediately leave his new bride to see his old friend). As Portia is insistent on having everything turn out her way, she sets out after the men with Nerissa in disguise to see what they can do about Antonio's case. Not only is the case complicated, but this further confusion of the cross-dressed women is sure to add some drama.
The play builds to the point where Antonio is practically shirtless in anticipation of the knife. Need we say more?
The Christians of Shakespeare's time would have seen Shylock's ending as a cliffhanger. Sure, he's been ruined, but will he find salvation in his new religion (Christianity)? Meanwhile, Graziano has stopped Portia on the street and handed over Bassanio's ring. Nerissa wrings Graziano's ring out of him too. The women have just done a great (if secret) service to the men by saving Antonio, and they can rightfully expect the men's gratitude and loyalty in return. Instead, the men give up their rings to the seemingly random doctor and clerk. This shows that they put their friendship and their own affairs above their wives, and they might be faithless in marriage after all. We've yet to see how the ring situation will pan out and whether the women will forgive their men for breaking their promises and offering up their lives to save their friend. The last task is for the women to get home before their husbands and remind everyone to keep their absence a secret so they can further toy with the ignorant minds of their menfolk.
There's a merry greeting at Belmont, and everything seems to be winding down until Graziano and Nerissa get in a squabble over the ring issue. Portia lets on that she won't let this get out of hand, which keeps us nicely calm and denouement-y as she eggs Bassanio on to admit that he too has given away his ring. As Portia is rather lighthearted about the whole affair (she promises to sleep with the doctor, which we know is impossible), we get the hint that the whole thing might not end too badly. Most important, this squabble gets Antonio to finally apologize, as he's been at the root of this particular trouble (and arguably would have been a pain through Bassanio's whole marriage). When he comes around to Portia's side by pledging Bassanio's faithfulness to her, we know everything will be OK.
Portia finally reveals the whole gig by presenting letters that explain how she was Balthazar and Nerissa his clerk. She accepts Antonio's apology and goes even further, ensuring that, even if he's lost his friend, he's got his money back. (This is where she delivers the news of his ships' success.) The whole play ends in relative harmony and all the marriages are consummated (though Antonio and Shylock are left in the proverbial cold).