The Lost episode centered on Desmond Hume and the idea of a fixed time event is “The Constant.” It is the fifth episode of the fourth season and is widely regarded as one of the series’ standout episodes.
In the story, Desmond’s consciousness moves between 1996 and 2004 after exposure to an unusual electromagnetic effect. Daniel Faraday helps him establish a connection between the two periods, while Desmond must find a person who exists in both times.
That person is Penny Widmore, whose phone call gives Desmond the emotional and temporal anchor he needs. “The Constant” is often remembered for combining science-fiction mechanics with a relationship story. It is distinct from “Flashes Before Your Eyes,” another Desmond-focused episode involving unusual experiences with time.