At her house in Belmont, Portia pleads with Bassanio and reveals her preference for him. She asks him to hang out with her for a month or two before he takes the casket test, as she'd rather have him around for a while before he's forced to leave her company if (when?) he makes the wrong choice. She carefully says her feeling isn't love, but also she wouldn't have such feelings if she hated him. (It's the Shakespearean equivalent of "I sort of don't hate you so maybe you want to hold hands at recess?")
Portia quibbles on what's just, given her father's will. She'd like to teach Bassanio how to interpret the challenge (read: cheat) so he'll choose the right casket, but that would be breaking her oath. Of course, if he makes the wrong choice, she'll sin anyway by wishing she had broken her oath.
Portia does some more fawning and swooning over Bassanio, and he simply replies that he'd like to take the test now, as all this waiting around is like being on the rack (a medieval torture device used to stretch people until they broke).
Portia plays along and asks what treason Bassanio has committed that he deserves the rack. Bassanio clarifies quickly: he says he's guilty of mistrust – he is fearful to love Portia fully now, as he can't trust that he'll be around much longer after the casket test. Still, he says, there's nothing off or untrue about his love for her.
Bassanio then decides he's had enough beating around the bush. He confesses that he loves Portia dearly, though actually it's the torture type of love.
Bassanio finally gets to pick a casket. Portia has everyone back off so he can choose in peace. She insists that music be played, so if he goes out, he can do it like a swan, but if he stays, then they've already got music playing for their party. How nice!
Portia then compares Bassanio to Hercules (whom she calls by another of his names, "Alcides"). She says Bassanio goes with no less presence but far more love than Hercules did when he had to rescue the virgin paid up as tribute by Troy to a sea monster. Portia says she herself is the sacrifice, while everyone else is like a Trojan wife, standing around to see the show. Portia says if Bassanio outlives this test, then she will live again.
While Bassanio is reasoning to himself about the caskets, there's a bit of a song (from the background, it seems, like in movies) about how one comes to fancy someone and whether that fancying is about the heart or the head. Once the musical interlude concludes, Bassanio lays out his reasoning, just as the other suitors have done.
Bassanio begins by saying he knows that what's on the outside is often deceptive about what's inside. Bassanio proceeds to list off a couple of instances where there are tricky ornaments that might convince you a bad thing was actually a good thing.
Bassanio does some more philosophizing, adding beauty to the list of transient ills. Women can wear makeup, and who wears most is the least prized. Ultimately, beauty is usually veiled, and outer beauty can hide inward ugliness.
Using all this reasoning, Bassanio makes his choice. He dismisses the gold casket as the gaudy food of Midas that proved inedible, and the silver as the paler of the two metals that are both made base as coins of money. That leaves him the lead casket, which he admits is threatening, but moves him more with its paleness than the eloquence of the other two precious metal caskets.
He hopes he's right and Portia, knowing he's made the right choice, is secretly flipping out with joy. All of her other passions have given way to love, and she now worries she feels the emotion in excess. She pleads with love to make itself felt moderately, as she worries she'll have too much of it at this rate.
Meanwhile, Bassanio has opened the lead casket and found Portia's portrait. He waxes on about how beautiful the picture is, but then reprimands himself for praising the picture, which is only a shadow of the real woman's beauty. (Now it seems a little ironic that he just gave a lengthy speech about looks not being important.)
He then reads the scroll inside, which congratulates him for choosing correctly; though it was chance, the chooser's decision to look beyond what was immediately apparent has rewarded him. The scroll then insists that he kiss his new wife.
Bassanio says he's shocked by his victory – right now he feels like a guy who's won a contest and only believes he's won it because everyone else around him is so happy. He's too shocked to believe this could really be happening, so he will only accept it as true once Portia accepts him.
Portia, of course, is stoked. She wishes she was better, prettier, and wealthier, so as to further please her new man. Though she admits she lacks experience, she's excited that she's neither too old nor too stupid to learn. She then gives herself over to Bassanio, happily accepting him as "her lord, her governor, her king." She says everything that was hers is now his, including her mansion, her servants, and herself. She gives him a ring to signify their new union and says that if he loses it or gives it away, it's as good as ruining their love.
Bassanio, again, claims he's too stricken by all of these new developments to say anything meaningful. He adds that his feelings are like the cheering of a crowd after a good prince's speech: nothing distinct can be heard, but all joy is expressed without any specific expression. He declares that if the ring ever leaves his finger, he might as well be proclaimed dead.
Then Nerissa, speaks up and congratulates them. Graziano adds his two cents, namely "Great, you're married. Now can I marry Nerissa?" Graziano explains that while Bassanio was focused on Portia, the lady, he was focused on Nerissa, the lady's lady. He's made his mouth dry from so much swearing of love.
Bassanio and Portia confirm with Graziano and Nerissa respectively that their love is all set, and lo and behold, two weddings are planned in one scene. (Way to economize, Shakespeare.)
Lorenzo, Jessica, and Salerio then randomly show up in Belmont. They're welcomed by everyone, but sadly, Salerio brings bad news from Antonio. He hands Bassanio a letter from Antonio and declares he can find out exactly how Antonio is doing from its contents.
Portia notes that as Bassanio reads the letter he grows pale and horrified – she's sure that its contents declare the death of a dear friend, as nothing else could provide cause for a man like Bassanio to suddenly look such a mess. She insists that Bassanio tell her what's in the letter, as she is now half of him and should share his grief as well as his joy.
Bassanio now has to admit the whole truth: he says he told Portia the truth when he said he was a gentleman by blood only, not by wealth. Even that, though, was stretching it, as not only did he have nothing, but he had bet his friend Antonio's life to get what little he needed to woo her. The letter from Antonio declares that all of his ventures, to every port, have failed.
Salerio says that things are even worse than they seem. It looks like even if Antonio had the money to pay back the debt, Shylock would insist on the original terms of the agreement for a pound of Antonio's flesh, money be damned. (It seems as though, since the money was specifically lent against Antonio's investments in his ships, the ruin of the ships means the bond is broken. Antonio isn't left with the option of procuring the money to pay Shylock back by some other means, such as his best friend's rich new bride's fortunes.)
Salerio says he's never seen a more greedy and inhuman man than Shylock, who has been bothering the Duke morning and night about the fact that justice must be served.
The tale continues: twenty merchants and the Duke himself have tried to reason with Shylock, but he's not hearing any of it. He wants justice for Antonio's breaking the agreement, and justice means he gets to carve Antonio up like a roasted turkey. Jessica confirms this story, and says it looks like curtains for dear Antonio.
Portia asks if all of this means Bassanio's friend is in trouble, and her husband-to-be confirms that Antonio, his dearest friend in the world, as noble as the ancient Romans, is screwed for 3,000 ducats.
Portia offers to pay off Antonio's debt twenty times over; no harm should come to Antonio because he is a friend of her new man. Still, though, she'd like to get first things done first – Bassanio should go to church with her so they can get married, and then he can go to Venice to take care of other business.
She says she and Nerissa will live like widows and virgins until everything in Venice is resolved. Still, she instructs Bassanio to have a good time with his buddies who have just shown up. Done with all her directions, she tells Bassanio to read her Antonio's letter.
In the letter, Antonio basically says, "I'm screwed, and I owe Shylock my life, which he'll have. Please come and visit me before I die, since I'm sacrificing my life for you. Love, Antonio." Portia then instructs Bassanio to go as soon as he's done his business in Belmont. Bassanio says he'll go, but he won't rest while they're apart.