A member of a landing party investigating the ruins on a collapsing planet contracts an unknown infection and returns it to the Enterprise, where it spreads rapidly by touch. Lt. Riley locks himself in engineering and shuts down the engines, which may be needed to get the ship away to avoid damage from the planet's impending destruction. Kirk slowly begins to lose control, and even Spock is affected by the infection, while the planet's final phase of collapse begins with very little warning.
