It’s the end of the world as we know it, but Ellie feels fine. HBO’s The Last of Us debuted tonight, and while the 90-minute premiere did plenty to explain this zombie post-apocalypse, if you haven’t played the games you probably have lots of questions. For example: what’s the deal with Ellie’s big reveal at the end of Last of Us Episode 1?

Here’s what you need to know to understand the ending scene of The Last of Us’ first episode. Spoilers ahead!

The Last of Us Episode 1 ending explained

The Last of Us takes place in the year 2023, two decades after a zombie-like fungus destroys human civilization. The story begins in Boston, now a military-controlled zone where survivors live under totalitarian rule. Joel (Pedro Pascal) is a smuggler who’s hired to bring Ellie (Bella Ramsey) out west… for unspecified reasons.

In the episode’s final moments, they make it out of Boston but encounter a soldier who threatens to arrest them. But first, he follows protocol by testing them for zombie infection. Suddenly, Ellie stabs the soldier, and when he prepares to shoot them all in response, Joel lunges at the soldier and finishes the job.

But here’s the final twist: It turns out, Ellie tested positive for a zombie fungus. However, it’s not that simple. She quickly explains that it’s been three weeks since she was infected and she’s not showing any signs of becoming a zombie. (Earlier in the episode, it’s established it only takes a day at most for the infection to kick in.)

In other words, Ellie is immune to the infection that’s completely destroyed civilization. Could that be why Joel was asked to smuggle her out of Boston?

To answer that question, we’re going to have to dive into the original game that inspired the show. So consider yourself warned: There are spoilers for The Last of Us the video game (and probably the entire show) ahead.

The Last of Us Episode 1 ending explained: What happens next?

In the video game, it’s further explained that Ellie really is immune to the virus. The hope is that whatever made her immune could be used to create a vaccine and save the world.

With Joel’s help, Ellie reaches a secret medical facility where they conduct tests that confirm her unique case could hold the key to some sort of vaccine. However, the only way to do so would mean doing lethal surgery on Ellie’s brain. In response, Joel refuses and shoots everyone before escaping with Ellie.

As for exactly why Ellie is immune, that part is never fully explained, but there are plenty of theories. The most common is that the fungus that infected Ellie actually mutated in some way (meaning her fungus, and not Ellie herself, is special). Another option is that Ellie really is special or perhaps has some sort of brain tumor that protects her. Some people also think that one of her parents was somehow infected when she was either conceived or born, giving her the equivalent of a pre-natal vaccination.

HBO’s The Last of Us has already shown us parts of the story that the game never did (like that scene in the 1960s), so it’s possible we’ll learn exactly why Ellie is immune. But for now, the key thing to remember is simply that she is immune, and that’s what makes her so important.

The Last of Us airs Sundays on HBO and HBO Max.