Enterprise picks up a distress call from a Vulcan ship, the Seleya, which T'Pol recognizes as a ship she once served on. The ship is located in a somewhat treacherous asteroid field — the asteroids are moving in a chaotic, unpredictable pattern due to the Expanse's spatial anomalies.
Archer, Reed, T'Pol and Corporal Hawkins set out for the ship in a shuttlepod and are encouraged when there appear to be bio-signs onboard. Once they board the Seleya, however, it appears eerily deserted. Apparently, the Vulcans were attempting to line the ship with trellium, but didn't get very far. As the team continues to explore the ship, they are surprised by several homicidal, zombie-like Vulcans who attack them. The crew manages to fend them off, but the strangely altered Vulcans block them from the airlock, cutting off their escape route to the shuttlepod. What's worse, T'Pol seems to be suffering from the same symptoms as the crew of the Seleya.
Meanwhile, Trip and Mayweather are attempting to mine some of the asteroids for trellium ore, which they can use to protect Enterprise from the spatial anomalies. They start by beaming aboard various debris and then decide to take a shuttlepod to one of the larger asteroids. While attempting to collect the trellium, they are nearly crushed by a massive asteroid and barely manage to escape with their lives. The shuttlepod also takes some damage, and the two officers pilot it back to Enterprise for repairs.
On the Seleya, T'Pol's condition continues to deteriorate — she is becoming paranoid and is having a difficult time controlling her emotions. Once the team reaches an auxiliary control room, Archer contacts Enterprise and tells Trip to initiate a rescue attempt. Archer also transmits some scans of the altered Vulcans back to Phlox, hoping for some clue as to what caused their condition. T'Pol and Reed, meanwhile, work to gain control of the ship and un-block the path back to their shuttlepod. T'Pol, however, is becoming more emotional and paranoid by the second, and even accuses Archer of trying to kill all of the Vulcans onboard. She pulls a phase-pistol on him, but he manages to wrestle it away from her.
On Enterprise, Phlox is getting some idea as to why the Vulcans on the Seleya have become violent. Apparently, trellium-D is a potent neuro-toxin to the Vulcan nervous system and causes them to lose control of their emotions. Phlox contacts Archer and tells the captain that he can still treat T'Pol, but he needs to start as soon as possible. Unfortunately, the other Vulcans have been exposed for too long — their neurological damage is irreversible.
Suddenly, the com system on the Seleya is shut down — the altered Vulcans have turned it off and are now flooding the auxiliary control room with hexaflourine. Luckily, the officers manage to escape, but T'Pol is now completely delusional and resisting Archer every step of the way. Archer and the others make a break for the shuttlepod, fending off murderous Vulcans along the way and crossing a treacherous chasm in order to reach their ship. Archer finally stuns T'Pol with his phase-pistol, carrying her the rest of the way. Once the team reaches the shuttlepod, they are unable to release the docking clamps. Luckily, Trip arrives in Shuttlepod Two, and blasts apart the clamps, freeing the other pod. The two shuttelpods manage to escape just as the Seleya explodes. Back on Enterprise Phlox treats T'Pol successfully. When she realizes that trellium was the source of her condition, she tells Archer to leave her on the next hospitable planet and continue his mission. Archer, however, refuses — he asks Trip to store the trellium they've collected in a bio-hazard locker and informs T'Pol that he won't leave any of his crew behind.