After Hoshi and Trip contract a deadly virus while exploring a planet, two ghostly aliens possess the bodies of various Enterprise crew members to observe humanity's reaction to imminent tragedy. As the aliens secretly jump bodies to observe, Phlox strives to cure the stricken pair, but with his efforts failing, Archer must make a difficult decision in order to shield the rest of the crew from this growing biohazard.

• The non-corporeal aliens are the Organians, who first appeared in the Original Series episode "Errand of Mercy".