Fifteen years ago, a television show changed Network TV and serialized genre storytelling forever. Combining Survivor with Twin Peaks, Lost quickly became a global phenomenon. It made its large cast overnight superstars, and it inspired dozens of copycats with its use of flashbacks, and its character-first approach to the story that was still sprinkled with a fascinating mystery involving smoke monsters, polar bears and eventually time-travel. We’re still living in the shadow of Lost, and when you watch Game of Thrones or This Is Us, you can still see its influences.

But what we’ve come to associate with Lost weren’t exactly there from the beginning. The pilot used flashbacks, sure, and episode two started the tradition of focusing on a single character’s flashback each episode. However, it wasn’t until episode three, ‘Walkabout’, which aired 15 years ago this past weekend, that Lost truly cemented itself as a ground-breaking new player in Network TV. ‘Walkabout’ managed to combine the show’s characters-first storytelling with a rich mystery by introducing the mystical properties of the Island, and by using the flashbacks to really flesh out the characters and take the audience by surprise with memorable twists.

This article contains spoilers for all of Lost.

The episode centers on John Locke (Terry O’Quinn), who by this point the audience had come to know as a mysterious badass who seems to be the only one prepared for surviving on the Island, able to hunt and build, carrying around a suitcase full of knives. But the flashbacks tell a different story, as we see a Locke that actually was just a low-level box company employee belittled by everyone he knew, arguably the most tragic character backstory in the entire show. And of course, the last minutes of the episode reveal that John Locke was also paralyzed from the waist down before the crash.

Episode two, ‘Tabula Rasa’ didn’t really offer any new information on Kate, as we already knew she was the fugitive since the pilot episode. But ‘Walkabout’ was a huge revelation for fans, who were realizing that everything we thought we knew about the people on the Island or even about Lost was wrong, and that’s how it would be for the rest of the show’s run. As co-creator and co-showrunner Damon Lindelof said in an interview for VOX: “This character had been presented as this very sort of mystical figure moving forward. I think the thing that we were really excited about was this idea that we were presenting Locke as one thing, as sort of like the hunter, the survivalist, the guy who brought all the knives. But his first flashback was going to show you that he was just a cubicle jockey.” Indeed, the flashback changed how people saw John Locke, making him an even more fascinating character.

Before the show introduced conspiracies, secret organizations, time-travel and immortal beings, Lost was about a group of people who had to learn to live together in order not to die alone, with a few mysteries sprinkled on it to make the drama more interesting. Indeed, in the earlier days of the show, ABC was scared of the prospect of a genre show, so the creators had to make them believe it was simply an absolutely self-contained, episodic show that would be easy to drop in and out of and never get confused by the mythology. In that regard, ‘Walkabout’ perfectly encapsulates the early seasons of Lost, a show about a group of people coming together in an Island that may hold special properties.

Outside of Locke’s flashbacks, the episode deals with Jack and Rose talking about her husband Bernard and the possibility that the people in the tail section of the plane are still alive, and Claire putting together a memorial for those who lost their lives in the crash. Even something as simple as Sun caring for Walt while Michael joins Locke and Kate to go hunt some boars brings home the idea that these people are starting to build a community that will help them heal from their previous traumas, all of which starts right in ‘Walkabout’ – and lead the castaways to become so close they create a space in the afterlife for everyone to reunite. The flashback in this episode and subsequent ones would flesh out the individual characters, while the stories on the Island would develop the characters through their relationships with one another.

‘Walkabout’ was also significant because, as the on-Island story began building the community that would become the core of Lost, it also deepened the mythology of the Island. Towards the end of the episode, Jack sees a man in a suit walking into the jungle, who would later become a big part of the show and central to Jack’s arc, and more importantly, Locke comes face to face with the monster.

This last bit is vital for the rest of the show, as Locke looking at the monster everyone else was afraid of and not even flinching – together with his miraculously healing upon the crash – informs Locke’s arc as the “man of faith” who becomes obsessed with the Island and is ultimately fooled by forces greater than him. Even from the pilot episode, Locke is in many ways Lost’s first real mystery. When we first meet him, he is just an enigma in the form of a symbolic man that we see smiling with an orange peel obscuring his teeth, the only survivor who welcomes torrential rain with open arms as the rest of the castaways run to find shelter. Even before we see that he was magically healed by the Island, the audience is already wondering who he is and what he knows. In episode five, ‘White Rabbit’, Jack thinks he’s hallucinating his dead father, introducing the “man of science” versus “man of faith” dynamic that would follow them for the rest of the show.

Throughout the flashbacks in ‘Walkabout’ we see how Locke was always obsessed with the idea of fate and destiny, sure that there is something special waiting for him. As the show goes on and even more tragic events are added to his backstory, we understand how Locke sees the pains and setbacks in his life as proof that something better is coming to him soon. Then he gets confirmation that he is indeed special, as the Island chooses him to be healed of his paralysis. He seems to easily find the mysteries of the Island – even coming face to face with its version of the devil and coming out the side thinking it was beautiful. This becomes central to Locke’s character arc, but also to the show’s larger mythology, as it starts planting the seeds of the chosen people of the Island (first Locke and Walt) which would lead to Jacob and the lighthouse in season 6.

By ending ‘Walkabout’ with another flash to the crash from the pilot episode but from Locke’s perspective, the episode recontextualizes this grizzly and painful ordeal with blood-curdling screams and turns it into a magical birthing scene wherein a disabled man becomes the person he always wanted to be. For the rest of the castaways this was the worst thing that could happen to them, but Locke this was the best, and it became his mission in life to protect this place that saved him from his previous life – even if it meant sacrificing his life for it. Locke’s arc as he becomes a sort of antagonist, is realizing that he got the wrong message about faith, thinking there is something bigger than him guiding things and elevating him. On the other hand, Jack realizes that the “something bigger” is just the people around you that elevate you. Locke becomes a zealot for the Island, without knowing that in reality he’s just a pawn in a game played by two beings we don’t even know yet.

In the grand scheme of things, ‘Walkabout’ may seem like a smaller episode, but thematically and narratively, this episode encapsulates everything Lost would be known for in just under one hour. The flashbacks contributed to one of the show’s main themes of being able to redefine yourself to be exactly what you want to be, while the twist contributed to Locke’s “man of faith” narrative and also deepen the mythology of the Island, while the main story started building the community that would become the center of the show’s story. Even 15 years later, this is the most important episode of Lost, and the one that proved this show was unlike anything that had come before it.