In ABC’s Lost show, Desmond Hume (Henry Ian Cusick) possessed unique time travel abilities. Despite not being one of the Flight 815 passengers, Desmond turned out to be one of the show’s most important characters. Much of his significance to Lost’s story can be attributed to his ability to travel to different time periods.
Originally, time travel was one genre that had no role to play in Lost. However, that began to change when Desmond became a main character. In a season 3 episode, Lost moved in a brand-new direction by implying that Desmond remembered being on the island during his 1996 flashback story. Of course, that seemed impossible since he hadn’t arrived on the island at that point. Then in season 4, Lost took things to the next level by putting Desmond’s consciousness in the body of his 1996 self in the critically-acclaimed episode, “The Constant”. Desmond’s discovery of time travel paved the way for the main characters’ own experience with the concept in Lost season 5 when the island became displaced in time.
Desmond being such a special case among the show’s cast of characters can be attributed to the events of the season 2 finale. When Locke and Desmond stopped Charlie and Mr. Eko from pressing the button in the Dharma facility, it triggered a massive release of electromagnetic energy. Preventing its release turned out to be the purpose of the Hatch and the numbers that had to be entered into the computer. To reverse their mistake, Desmond activated a failsafe to undo the process, and seemingly sacrificed himself to save everyone else. Having been so close to the explosion that destroyed the Hatch, Desmond was exposed to a tremendous amount of electromagnetic energy. Though he didn’t know it at the time, this event set in motion his biggest Lost stories.
This moment was responsible for his premonitions of the future. Presumably, the visions in Lost season 3 were a result of Desmond’s mind being briefly sent forward in time, allowing a peek at events that had yet to happen. But most importantly, the electromagnetic radiation his body took in was the primary cause for the temporal displacement that Desmond occasionally suffered from in the series. His body never left his place in the timestream, but his consciousness was able to move to different points in his own timeline. In order to survive (and end) this phenomenon, he had to find a “constant” in his life that existed at both times in his life – Penny.
Characters such as Daniel Faraday and Charles Widmore understood how truly special Desmond was. Because of everything that happened to him, he had developed an astounding resistance to electromagnetic radiation, which was why he was used by the villains to find the Source. These unique properties possessed by Desmond effectively made him the key to Jack reaching the Source, the character getting to the afterlife, and in turn, Lost’s ending in general.