The Doctor and Mel strive to find a solution as the Vervoids rampage. Ruth Baxter, once human, is revealed to be partially transformed by the Vervoid biology. Lasky attempts negotiation but is betrayed; the Vervoids claim to have no alternative but to kill—or be killed. On top of that, mutinous activity among the crew and alien collaborators threatens the ship. The Doctor works toward a radical plan involving rare metal vionesium.