As surprising as it may be, season 4 of Lost offered the best time travel story that’s ever been on TV, despite the fact that Lost wasn’t even a time travel show. The concept was introduced to the show in the season 3 episode “Flashes Before Your Eyes” and evolved into a major plot device in season 5 when several of the Oceanic Flight 8-15 survivors became trapped in the 1970s.

Prior to the show’s big time travel twist, Lost aired the Desmond-centric episode “The Constant” in season 4. In “The Constant,” an incident causes Desmond’s consciousness to travel back in time to a distant point in his past. Desmond finds himself in 1996, while his 1996 self takes his place in the present. While in the past, Desmond has to seek out Daniel Faraday (Jeremy Davies) who helps him figure out how to survive and make things go back to normal.

Time travel stories are a dime-a-dozen in science fiction, but “The Constant” provides viewers with a unique and original take on one of the oldest and most popular concepts in science fiction. Desmond traveling back and forth between two different points in his life is almost like Quantum Leap, but with a huge twist. After finding out that the constant time travel is killing him, the story receives a new sense of urgency, with Desmond discovering that the only way for him to live is for him to find a “constant,” a person or thing who exists in both times who can “ground” him.

The idea behind the “constant” is another fascinating concept that’s unique to Lost. All of this leads to a gripping scene where Desmond goes looking for his ex-girlfriend, Penny (Sonya Walger), in 1996, and begs her to give him her number. Immediately afterward, Desmond flashes back to 2004 and is able to call her. The scene where Desmond, through the power of time travel, is able to make contact with the love of his life is easily one of the show’s most memorable moments. The call proves to be just enough to save Desmond’s life and restore his consciousness to its proper place in the timeline.

Not only is it TV’s best time travel story, but it often tops the lists of the best Lost episodes — and for good reason. “The Constant” is time travel at its best, and Lost delivers the story in a rare, perfect episode of television.

More: “Not Penny’s Boat” Is Lost’s Biggest Moment (Not “We Have To Go Back”)