Lost is about the survivors of Oceanic flight 815, but only two of them actually appear in the show’s greatest episode. Lost season 4, episode 5, “The Constant,” delves into time travel and its consequences when present-day Desmond Hume (Henry Ian Cusick) suddenly starts flashing back to his younger self in 1996. His mind flicking between the past and 2004, Desmond must figure out how to anchor himself in the correct timeline before becoming another victim of the notoriously turbulent Lost cast.
https://screenrant.com/lost-constant-best-episode-plane-crash-characters-missing/