Two Organians possess members of the crew to watch how they respond when Sato and Tucker contract a deadly silicon-based virus.
Two Organians, non-corporeal beings who have spent centuries testing how spacefaring species respond to a deadly silicon-based virus, take over the bodies of Enterprise crew members to watch. Tucker and Sato are infected on an away mission and isolated in decontamination.
Phlox and T'Pol attempt a dangerous radiation treatment, and Archer removes his helmet to help Sato, but she and Tucker both die. Archer argues that the Organians have lost their compassion, and the observers change their protocol, reviving and curing the pair before erasing the encounter from the crew's memories. The episode was directed by Mike Vejar and written by Judith and Garfield Reeves-Stevens.