Calymath finishes inspecting the island's fortifications and states that he is confident it cannot be conquered. A messenger brings an invitation from Barabas, inviting Calymath to a feast at Barabas's "homely citadel." Calymath says that he does not want to bring all his soldiers to this banquet given that the town has recently been pillaged. The messenger replies that Barabas has a "pearl so big" that its worth would feed the troops for a month. When Calymath wonders how Barabas will fit all the soldiers inside the city walls, the messenger replies that the troops will eat in a monastery outside of the town; only the "Selim" and his "Bashaws" will feast at Barabas's house. Calymath agrees to the plan and says that they will have the feast that evening.
In this short scene, Ferneze tells his knights to come to his aid only when they hear a cannon fired. A knight replies that they will risk anything to avoid living "as Turkish thralls."
Barabas enters with a group of carpenters, who have just built some kind of apparatus that will be used to kill Calymath. Barabas pays the workers and tells them to go and drink some wine that he has poisoned. After they leave, the messenger appears and informs Barabas that Calymath will be feasting with him. Ferneze enters and gives the protagonist 100,000 pounds. Barabas explains how he has planted gunpowder underneath the floor of the monastery where most of the Turkish troops will feast. He also states that he has rigged the floor of the gallery of his home so that it will collapse, plunging Calymath into a "deep pit past recovery." Barabas instructs Ferneze to cut a cord holding up the balcony when he hears a trumpet sound. Barabas refuses to take Ferneze's money until his plan has been successfully executed. As the Turks enter, Barabas delights in his deceit saying, "tell me, worldlings, underneath the sun, / If greater falsehood ever has been done."
Barabas greets the Turkish leader and invites him and his soldiers to climb the stairs to the gallery. At this point, Ferneze steps in and warns Calymath not to move. A knight sounds a trumpet, the cord is cut, and Barabas falls through the trapdoor he himself created into a cauldron below. Calymath is bemused, but Ferneze explains that Barabas's fate was meant for him. The Turkish leader cries "treason" and prepares to leave, but Ferneze persuades him to stay and watch Barabas die. Neither man helps Barabas. Barabas confesses to his crimes against Ferneze and Calymath, and dies cursing "Damned Christians, dogs, and Turkish infidels."
Calymath prepares to leave, but Ferneze stops him. Ferneze explains how Barabas has killed all the Turkish troops feasting at the monastery. Ferneze tells Calymath that he cannot depart until his father "hath made good / The ruins done to Malta and to us." The Turk remonstrates, but Ferneze stands firm and says that the Turk is now his prisoner. Ferneze concludes the play by stating "let due praise be given / Neither to fate nor fortune, but to heaven."
The closing scenes of Act V are filled with drama and move quickly towards the play's unexpected denouement. Ferneze appears ready to move ahead with Barabas's plan as he instructs his officers to wait for the sound of cannon-fire before attacking. Similarly, the protagonist is preparing his house for a final scene of mass destruction; his decision to poison the carpenters just seems like a warm-up for the planned climax.