The Leftovers just pulled off its trippiest episode yet -- and coming in a season that already featured a lion sex cult boat party, that's saying a lot.

In Sunday's episode, Kevin Garvey (Justin Theroux) voluntarily has himself drowned so he can return to the Afterlife Hotel, only instead of the hotel he previously visited, he wakes up on a tropical island. Suited up in the familiar black suit, Kevin's back to being an assassin whose new mission is to kill the President of the United States. However, as soon as Kevin sees his reflection, he becomes the president, who just so happens to be a leader of the Guilty Remnant.

When Theroux first read the script for "The Most Powerful Man in the World (and His Identical Twin Brother)," the actor says he was "blown away" by the writing. "One [reason] was I was going to be back in that realm again," Theroux tells TVGuide.com. "And then in a weird way, it's sort of a bookend to the first 'International Assassin' episode where, essentially, his journey was to walk this person to the end of their life and kill them or put them out of their misery. And then this one, he has to do it for himself, and so that I found fascinating."

The decision to move the action away from the hotel was a smart one, Theroux says, not only because it kept the show from being repetitive, but because it opened up new means to externalize Kevin's inner-most conflicts. "I think they wanted to just find a new dynamic and empower Kevin, essentially, and make him the most powerful man of his destiny," Theroux says. "So the fact that he is the most powerful man in the world as president is just a device, really, for a metaphor for what he needed to do to fix himself."

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"It just made sense that Kevin's greatest enemy is this part of him that's broken," adds co-creator Damon Lindelof. "Even though the two Kevins are seemingly both really good guys, one of them is trying to destroy the world and, in fact, succeeds in every villains' mission... The bad Kevin actually wins, but he's just trying to help out the good Kevin."

But not every decision made in the show-stopping episode was done with symbolism in mind. For anyone racking their brain for the deeper meaning behind Patti (Ann Dowd) being Kevin's Secretary of Defense or Meg (Liv Tyler) being his Vice President, you're wasting your time if you think the answer is anything beyond, "because it would be cool."

As Lindelof simply explains: "I love sequels where you have to team up with the villain from the last movie in order to take down the new villain."

In the end, both Kevins do decide to go along with Secretary of Defense Patti's plan to destroy the world. Assassin Kevin nobly sacrifices himself to President Kevin, allowing the beautifully bearded POTUS to dig out the key embedded in beardless Assassin Kevin's heart that will allow the plan to move forward. As President Kevin does the bloody work, killing Assassin Kevin in the process, there's a sense that Kevin is finally seeing himself -- and the full consequences of his actions -- for the first time.

On what Kevin took away from the experience, Theroux says it's a sense of regret. "It's the notion that he had dug in and, having left Nora (Carrie Coon) for second time, he abandoned his family to an extent. The first time happens in Season 1 in Episode 9 in that flashback episode before the world falls apart," he explains. "He has that moment with his father where he says, 'This is it. Is this as good as it gets?' And his father's like, 'Yeah, this is it. This is the good stuff though.'

"And he regrets that decision to kind of step out once his family implodes, and he does it again in Australia when he walks out on Nora and basically says, 'enough is enough' and leaves her and realizes he's either afraid of intimacy or afraid of having exactly what he wants."

Unlike Kevin's previous trips to the Afterlife, this trip is poised to deliver real, positive change for Kevin. Before, Kevin believed that if he could get rid of his external antagonist, Patti, then he would be OK -- a notion that was quickly dismantled this season when when viewers caught Kevin recreationally suffocating himself. As this episode made clear, Kevin's biggest enemy has always been within, and now that he's no longer divided within himself, there's a growing hope that maybe -- just maybe -- Kevin can now allow himself to find a way to be happy again.

But more importantly, the episode delivered on an incredibly important prerogative of its showrunner: "We just wanted to make Justin have a beard and not have a beard in the same scene, really," Lindelof jokes.

The Leftovers series finale airs Sunday at 9/8c on HBO.