The USS Enterprise orbits the planet Psi 2000, a world that was much like Earth in its distant past, tasked to observe the planet's impending disintegration. Lieutenant Commander Spock and Lieutenant junior grade Joe Tormolen beam down in environmental suits to a frozen surface laboratory and investigate the horrific deaths of the lab's scientists. Carelessly, Tormolen removes a glove of his suit to better scratch his nose, unknowingly exposing himself to a red, blood-like liquid substance leaping to his exposed hand from a frozen wall. Spock contacts the Enterprise and informs Captain Kirk that all of the station's personnel are dead. Kirk asks what caused it and Spock replies, "Unknown, captain. It's like nothing we've dealt with before."