Leon, Claire, and Sherry prepare to leave the police station and go to the sewer system, but along the way they encounter the creature with the bulbous eyeball on its shoulder. Wielding a pipe, it attacks them and proves to be a formidable opponent. It begins to attack wildly, and eventually tires before falling over the railing and into the abyss. The bridge collapses from the strain of the fighting, and Leon falls down after him into the sewer. Upon recovering, he finds Ada again. She's trapped in a gun fight with another woman, who calls her a "thief". Ada plans to make a maneuver, and Leon can see she'll leave herself wide open. As Ada moves and the woman takes aim, Leon jumps between them and takes a bullet to the back. The woman runs off, and Ada tends to Leon's wounds. He inquires about the altercation, and senses Ada won't be as forthcoming as he wants her to be. He insists she tell him more, but she reluctantly declines as she views the contents of a familiar looking pendant.
Leon and Claire proceed to the P-4 Lab to make the vaccine. Along the way they see the handiwork of a Giant Moth, and see that its nest covers most of the area outside the lab. After making the vaccine, they depart, running past many B.O.W.s along the way. Upon returning to the walkway near the Security Room, a pipe suddenly strikes the fuse box nearby. It's William, still in his second. Leon and Claire discover their locked in, and engage him. Once he falls they attempt to leave. A Web Spinner appears but they kill it, and suddenly William is back. He's mutated again, into his third form. His third form proves to be strong and agile, with a long reach and multiple arms. Leon and Claire fight him off and he stumbles over the railing and into the abyss. They turn to leave again, but suddenly they hear a monstrous roar. Its William yet again, this time in his fourth form. Now a larger hexapedal creature with a huge gaping maw of fangs, he proves to be a more formidable opponent than before. Leon and Claire find it difficult to fight him, as his immense weight begins collapsing parts of the walkway. Soon, they have him hanging from the edge, but he hangs on defiantly. Leon jumps down to loosen his grip and William falls into the abyss.