This guide will show you how to complete the mission to clear the town of bandits at Fort Mercer in Red Dead Redemption. The quest will only be available after you help the madman Seth find the so-called treasure. Head to the John Marston point on the map, where Irish, Dickens, and Marshal Johnson will be waiting for you.
Seth managed to sneak inside Fort Mercer, so you need to hurry before he gets thrown out. His task is to distract the bandits so you can get inside. Get into the carriage and wait for Dickens to bring you to the fortress entrance. Wait for the end of Dickens' dialogue with the bandits and shoot from the previously prepared machine gun at Williamson's men. Be attentive and quick to avoid catching a stray bullet and failing the mission.
After killing everyone inside the fort, your task is to chase the escapees who fled to the other side. After defeating them all, new enemy forces will arrive. This time — on horses. Shoot back at the newcomers and watch the cutscene. One of the bandits was caught by Irish, who revealed that Bill Williamson fled to Mexico near Chuparosa yesterday morning. After learning all the information, the mission will be considered complete, and you will receive +75 points to the Fame parameter.