We're at Barabas's place, and he's ordering a bunch of carpenters to build some sort of complicated pulley-rope-crane device.
After they've finished and left, Ferneze comes in, carrying a big sack of money he's put together from the Maltese citizens.
Like, really really big—100,000 pounds, in fact.
Barabas dishes on the plan:
There are cannons hidden beneath the monastery where Calymath's army is feasting, and at Barabas's signal the entire thing is going to blow up.
The floor of Barabas's house, where Calymath himself is going to be eating, is rigged so that when a cable is cut it drops down into a pit.
This last bit is Ferneze's job—he has to cut the cable at the right time.
Ferneze hides, and Barabas crows to himself about how very clever his plan is.
Calymath comes in, but before Barabas can do much Ferneze bursts out, telling Calymath that he's going to do better by him than Barabas would have.
Things are already set in motion, though—a Knight gives the signal to blow up the monastery, and the cable is cut.
It's Barabas, though, not Calymath and his bashaws, who falls through the trap door, and into a cauldron which has been placed below.
It would appear that Maltese cauldrons are sold in the sizes of Small, Medium, Large and Extremely Nasty Death For Adult Men.
As Barabas cries for help, Ferneze explains to Calymath that the trap was devised by Barabas and meant for Calymath.
Once Ferneze makes it clear he's not going to help Barabas, Barabas figures now's as good a time as ever to confess to killing his son.
While he's at it, he tells Calymath he wants to kill him too—in fact, he wants to destroy all the Christians and Turks.
The cauldron is on fire, presumably (we don't actually get a stage direction for this, but Barabas complains of "the extremity of heat," so why not?), and having cursed both Christians and Turks alike, Barabas finally dies. For real, this time.
Ferneze, repeating that he's done Calymath a big favor by not killing him, tells Calymath that he has to stay in Malta—after all, all of his Turkish men are dead.
Say what? Yep, that's right. That sound you heard? Exploding monastery. Sorry, bros.
Ferneze's going to hold Calymath hostage in Malta until his father, the Emperor of Turkey, makes reparations and frees Malta.
Ferneze closes the play by swearing he won't let Calymath go until his demands are met, and thanking heaven for his victory.