If there’s one thing “Lost” fans can agree on, it’s that season 4’s “The Constant” was a standout episode. (There’s a reason it’s right on top of /Film’s “Lost” episode ranking.) Following fan-favorite Desmond (Henry Ian Cusick) as his consciousness is stuck jumping back and forth between 1996 and 2004, the episode makes for a fun, mostly self-contained sci-fi adventure that suddenly transforms into the most heart-wrenching romance you’ll ever see. That lovely phone call between Desmond and Penny at the end is beautiful, and it’s made even more satisfying by the realization that the show had been building up to it since the season 2 finale.

“Lost” co-creators Damon Lindelof and Carlton Cuse consider “The Constant” to be one of their finest hours, although it was also one of the most difficult to write. Speaking at the San Diego Comic-Con in 2009, Cuse described the episode as one that took a long time to brainstorm and execute well. It was such a tiring process, in fact, that when the 2007-08 writers’ strike came and shook up the production of season 4, it was almost a relief for them.

“Normally it takes us a couple weeks, usually two weeks, to break a ‘Lost’ episode, and that one took us five weeks,” Cuse explained. “In fact, the strike kind of came along at just the right time, because we had sort of exhausted all of our lead time, depleted all of our resources breaking that story, we were running right behind production […] we were completely sorted of zapped, and it was a good time to go on strike.”

“That was an episode that was almost impossible to explain to somebody,” Lindelof said. “We remembered having conversations saying, ‘Give us more time, but Desmond’s consciousness from the present on the way to the freighter gets misplaced, so he’s back in the early ’90s when he’s still in the military, but he’s completely confused as to where he is or when he is, but at the end he’s gonna make out with Penny.'”

The episode changed a bit from the early idea to the final product; Desmond does not get to make out with Penny in the episode, for instance, although we’d argue that phone call was way more romantic. Still, this early explanation is a mostly accurate summary of the episode, and it’s easy to see why studio execs would be confused. The concept immediately brings to mind impossible-to-answer questions like “How does somebody’s consciousness travel through time but not their body?” and “Where exactly is 2004 Desmond’s consciousness throughout all this?” and “What did 1996 Desmond think about this after his part in the story ended?”

We never get a concrete answer to any of these questions, and it must’ve been hard to tell in the beginning just how little those lack of answers would matter. “Lost” fans are famous for badgering the writers for more answers to the series’ many mysteries, but for “The Constant,” everyone was down to go along for the ride.

The immediate positive reception to “The Constant” must’ve been a massive relief to Cuse and Lindelof, because this was the episode that marked a clear shift in the series’ approach to the sci-fi and the supernatural. Whereas the first three seasons were grounded and character-focused, with the speculative elements largely looming in the background, “The Constant” dove headfirst into straightforward sci-fi. Suddenly the show was traveling through time and casually breaking all known laws of physics. It’s the sort of decision that could have been a real jump the shark moment for “Lost,” but instead it only made the audience even more excited to see what came next.

Sure enough, “Lost” gets weird post-“Constant.” It’s not long after it that Ben Linus pulls a magic wheel that makes the island disappear, and by season 6 a big portion of the show is taking place in an alternate reality that turns out to be the afterlife. In season 5, half the cast gets yanked back and forth through different eras of the island’s history, almost like the island’s skipping through a playlist of its most eventful years on shuffle mode. “Lost” season 5 is a full-on time travel epic, and season 6 is a fantasy adventure story; they were both fun and divisive in their own special ways, but the groundwork for both was established here. 

Another fun fact about “The Constant” was that Desmond’s storyline also functions as a way to avoid too much exposition about the freighter. If things had gone according to plan, Desmond and Sayid’s visit to the ship could’ve easily fallen into the trap of giving viewers 40 minutes of didactic, lifeless explanations. As Lindelof explained in the behind-the-scenes DVD commentary, “You would have mindless exposition unless the characters were in a constant state of crisis.”

But “Lost” was never at it best when it was just straightforwardly answering one question after another (remember how stiff “Across the Sea” felt in season 6?), so here it chose to answer the audience’s freighter-related questions through little bits and pieces. The audience gets a ton of new information about the basic power structure on the freighter (as well as the weird time rift between the ship and the island), but we learn all this only through how it relates to Desmond’s immediate, tension-filled situation. That is to say: We learn a lot of important stuff here, but it never feels like homework.

The episode’s treatment of the freighter mirrors the treatment of Daniel Faraday (Jeremy Davies); there’s a ton of exposition about him here, but it never feels like exposition because it’s almost all told in relation to Desmond. Faraday would later go on to lead the thrilling season 5 episode “The Variable,” in which he’s also thrown into an improbable (yet compelling) time travel loop storyline of his own, and the groundwork of so much of that story was set up here, a full season earlier.

“We worked really hard to try to find a balance, that there was an emotional payoff to the story,” Cuse explained in that 2009 panel. “That we didn’t just spend the entire episode trying to explain our concept of consciousness travel, that it built to something that was emotional and genuine and kind of transcended the mythology of the episode.”  

It’s a statement that really cuts to the core of why the answer-demanding fans were so happy to go along with an episode that could’ve easily been nitpicked to death under slightly different circumstances. Time travel hijinks aside, “The Constant” is a love story, plain and simple. Even if you don’t understand the sci-fi elements at all, everyone understands on a gut level just how cathartic that final phone call between Desmond and Penny truly is. 

It’s the same approach that gave so many great “Lost” episodes their hall-of-fame status. The ending of “Walkabout” raises a ton of questions about the magic nature of the island, for instance, but what truly makes the scene shine is the sheer excitement of watching Locke (Terry O’Quinn) realizing he can move his toes again. “Numbers” might not explain what’s really going on with those numbers Hurley (Jorge Garcia) keeps seeing, but it does capture that sympathetic sensation of thinking you’re having a mental breakdown and being desperate to find someone who will acknowledge that what you believe in might be real. 

“Lost” was always at its best when it understood that it’s not the mysteries that matter, but the human drama that exists within them. That’s why over 15 years later, “The Constant” remains one of the best TV achievements of the 21st century.