Which Lost episode first revealed flash-forwards?

The Lost episode that first revealed flash-forwards was “Through the Looking Glass.”

The third-season finale, broadcast on May 23, 2007, follows the survivors’ struggle with the Others while intercutting scenes of Jack Shephard in a bleak future. He is bearded, addicted to painkillers, drinking heavily, and desperate to return to the Island. The episode initially presents these scenes like the show’s familiar character flashbacks.

The twist comes when Jack meets Kate at the end. Their conversation about leaving the Island, Oceanic Airlines, and needing to “go back” establishes that the scenes take place after the Island’s events shown in the main storyline. The revelation changed Lost’s storytelling format and became one of its most famous cliffhangers.

A common mix-up is “The Constant,” which is highly associated with time travel but does not introduce the series’ flash-forward structure. “Through the Looking Glass” is also a two-part finale, written by Damon Lindelof and Carlton Cuse and directed by Jack Bender.