With help from the Columbia, the crew stops sabotage aboard Enterprise, and Phlox finds a cure that leaves many Klingons with human-like features.
Sabotage by a Klingon agent leaves Enterprise's warp engines unable to drop below warp without destroying the ship, and the Columbia, under Captain Hernandez, comes to help. Tucker crosses between the two ships at warp on a cable to make repairs from the inside. Reed's secret dealings with a Starfleet intelligence contact, Harris, come to light.
Phlox, still held by the Klingons, works with the Klingon doctor Antaak to stop the virus. The cure saves the Klingons, but it leaves many of them with smooth foreheads and human-like features for generations, explaining the appearance of Klingons in the original series. The episode was directed by David Barrett and written by Judith and Garfield Reeves-Stevens.