In its sixth and final season, ABC's Lost shifted from its flashbacks and flashforwards formula to introduce a new device that hinted at the show's biggest twist yet: the flash-sideways. Lost was a groundbreaking series with a devoted following and blockbuster viewing figures. It featured an ensemble cast of characters trapped on a mysterious island after a plane crash left them presumed dead. However, beyond this central story, the series became notorious for its varied and complex narrative techniques, including the unorthodox flash-sideways.
Much of the mystery of the show centered around what the island truly was, why the passengers of Oceanic Flight 815 were drawn there, and whether escape was truly possible – even after some of the characters made it back home, each was fated to return, spawning the popular fan theory that the Lost island was purgatory. Lost used flashbacks from the first episode to inform the audience about how the characters' lives before the crash influenced their current situation, focusing on one character per episode, and weaving together a deeper understanding of the full ensemble over the course of entire seasons and the series as a whole. At the end of the season three finale, Lost introduced the flashforward when protagonist Jack Shepard was shown off the island, insisting to fellow survivor Kate Austen that they had to go back. The flashforwards juxtaposed the survivors' desperate escape attempts with their lives after leaving the island as they slowly realized their time on the island wasn't over.
The flashes sideways were used for a very different purpose. Flashes sideways were introduced in the sixth and final season, the most divisive of the show's run, particularly Lost's controversial series finale. While flashbacks and flashforwards are more or less self-explanatory, Lost's flashes sideways were glimpses into an alternate reality. The flashes sideways took place off of the island, contemporaneous with the timeline of the events of season one. The first flash-sideways began on the plane just before the crash, and snippets of dialogue from the original flight were repeated. However, in the flash-sideways, the Flight 815 crash and its aftermath never occurred. Instead, the plane landed safely and the passengers deboarded and continued on alternate reality paths off of the island. In this alternate reality, however, the passengers' lives became intertwined, allowing them to work through unresolved issues that had left them in various states of distress or unrest in the past.
As a device, a flash-sideways is an opportunity to compare the results of two potential courses of action (as in Avengers: Endgame) or a correctable mistake (as in WandaVision). Lost introduced the idea that mistakes in one reality can be corrected in a parallel one, and that events in one world can be mirrored in another. For example, in the real world, antagonist Benjamin Linus is given a redemption arc in the real world, but he only attains inner redemption through the flashes sideways. Linus murdered his father, crash survivor John Locke, and was responsible for the death of his daughter figure Alex. In the sideways world, Linus was united with all three. There, Locke was Linus's coworker and helped Linus resolve a professional issue. Linus now cared for the elderly father he killed. Most significantly, Linus was now Alex's teacher and was presented with the choice between sacrificing his future happiness or hers, paralleling the choice that led to her death. This time he prioritized Alex.
Closely examined, the flashes sideways were a big hint at where the season was headed. Lost's finale revealed that the characters died and were reuniting in the afterlife, which was consistent with the sideways world, where each of them tended to unfinished business and said final goodbyes. It was a fitting ending for a show that felt like a metaphor for purgatory (debunked by the creators) despite the fact that it was a controversial finale. In the end, all of the clues added up, and the flashes sideways were a journey to Lost's inevitable final resting place.