Admiral Ives orders Aubrey to sail to Zambra on the Barbary Coast to persuade the Dey of Mascara not to molest British ships, in convoy with HMS "Pollux", which is returning Admiral Harte to England. While "Pollux" waits at the entrance of the Bay of Zambra, the French "Mars" with two frigates fire on her, with a fierce ensuing battle. "Pollux" blows up, killing all 500 aboard, but not before she severely damages "Mars". The two frigates chase "Surprise" deep into the bay until the heavier frigate runs aground on a reef. Her smaller consort deserts the fight. On the political advice of Maturin, Aubrey sets sail for Gibraltar. This ambush on a voyage known to so few makes it clear that someone highly placed in the British command betrayed them to the French. Maturin hopes Wray will find the traitor out and destroy the French spy networks.