The crew finds a couple who are the lone survivors of an attack.
The Enterprise answers a distress call from the colony on Delta Rana IV, finding the planet destroyed except for a small cottage inhabited by an elderly couple, Kevin and Rishon Uxbridge. They claim to be the only survivors, and nothing appears to be wrong with them. But the attacks that wiped out the colony seem to be targeting the Enterprise, too.
There is more to the story than meets the eye, as the old man has a secret that is tied to a deadly alien race, the Husnock. The episode is a mystery that is revealed in stages, with a surprising and moving twist about the power of grief. It is a strong, haunting story, and one of the best episodes of the third season.