While playing a friendly game of chess, it becomes apparent that Reed and Mayweather have been inhabited by two members of an alien species who intend to observe how humans react when faced with an imminent tragedy.

While returning from a routine scavenger expedition on a mysterious planet, Trip collapses from a sudden pain in his lungs, forcing Hoshi to pilot the shuttlepod and make an emergency landing aboard Enterprise. Suddenly, Hoshi also becomes ill and Phlox delivers the devastating prognosis that Hoshi and Trip have been infected by an incurable virus that will leave them dead within hours, as well as anybody with whom they come into contact.

Hoping to ease their pain, Phlox injects Hoshi and Trip with a sedative, causing them to drift off into a deep sleep. Suddenly, Trip and Hoshi sit up in bed, inhabited by the aliens who now wish to experience the sensation of human illness. Phlox, stunned to see Hoshi and Trip awake and talking, gets the shock of his life when the aliens admit their presence in his patients. Phlox is soon silenced when the aliens erase his recent memory, as they have also done with Mayweather and Reed.

After the alien leaves Hoshi's body, she suddenly goes into cardiac arrest. Upon realizing that Phlox can't handle the defibrillator on his own, Archer throws off his gloves and lifts his bio-hazard helmet; thus exposing himself to the deadly virus.

Stoically, Archer accepts his dire fate and, and after placing T'Pol in command, goes to stay with Trip until the end. Trip, now inhabited by one of the aliens, sits up and confesses to a stunned Archer that he is an alien who has been occupying Trip's body in order to conduct research on humans. Archer is furious when the aliens admit to knowing about the deadly virus as well as having the power to cure those infected.

Archer begs the aliens to intervene and save Hoshi and Trip, but his ardent plea falls on deaf ears. Compassion wins out in the end when the aliens bring Hoshi and Trip back to life as a reward for Archer's heroism.