Enterprise finds a Vulcan ship whose crew has been driven mad by trellium-D, and T'Pol starts showing the same symptoms.
Enterprise answers a distress call from the Vulcan cruiser Seleya, which is drifting in an asteroid field rich in trellium. Archer, Reed and T'Pol board the ship and are attacked by its crew, who have become violent and zombie-like, and T'Pol begins to show the same symptoms.
The team fights its way to engineering and triggers an override, then races back to their shuttlepod, freed by Tucker and Mayweather just before the Seleya explodes. Phlox explains that trellium is a neurotoxin for Vulcans; T'Pol's brief exposure was not fatal. Archer has the trellium ore Tucker mined for hull shielding locked away. The episode was directed by David Livingston, with a teleplay by Jonathan Fernandez.