The Last of Us

The Last of Us Episode 9 Finally Solves the Game’s Biggest Mystery

So that’s how Ellie became immune.

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Liane Hentscher/HBO

The Season 1 finale of The Last of Us opens with a reveal that has been 10 years in the making.

Rather than picking right back up with Ellie (Bella Ramsey) and Joel (Pedro Pascal), the episode, titled “Look for the Light,” flashes back in time. The episode’s opening minutes follow Ellie’s mother, Anna (Ashley Johnson, who played Ellie in the original Last of Us games), as she tries to hide in a deserted farmhouse from a crazed, female Infected while also giving birth. In typical Last of Us fashion, things quickly take a tragic turn.

The season finale’s prologue reveals how Ellie was born on the same day her mother died and, even more importantly, how Ellie became immune to the very Cordyceps that has torn the show’s world apart. The episode’s game-changing reveal is particularly noteworthy because, despite how important it is to the series’ story, the source of Ellie’s immunity famously wasn’t revealed in either of Naughty Dog’s original Last of Us games.

In its Season 1 finale, The Last of Us finally resolves its biggest mystery.

Liane Hentscher/HBO

After running through the woods with a member of the Infected hot on her trail, Ashley Johnson’s Anna holes up inside a seemingly abandoned farmhouse in The Last of Us Episode 9’s opening flashback. She subsequently barricades herself in one of the house’s second-floor rooms and tries to finish giving birth to her daughter, Ellie, before her Infected pursuer can reach her. Unfortunately, not everything goes according to plan.

The Infected woman breaks into Anna’s room and attacks her. While she manages to kill her pursuer with the very switchblade Bella Ramsey’s Ellie treasures in the present day, Anna still isn’t able to fully enjoy the moment of her daughter’s birth. She quickly realizes that she was bitten on the leg by her Infected attacker. Panicking, she cuts the umbilical cord that ties Ellie to her and spends the next few hours holding her daughter in her arms.

That night, Marlene (Merle Dandridge) and some of her fellow Fireflies arrive at the farmhouse to discover Anna, Ellie, and the dead body of the Infected that Anna killed. Anna tells Marlene that she cut Ellie’s umbilical cord before she was bitten and then asks Marlene to find a safe home for Ellie and leave her switchblade with her. In case all of that isn’t tragic enough, Anna then asks Marlene to kill her before she turns into one of the Infected, which Marlene reluctantly does.

The Secret of Ellie’s Immunity

The Last of Us Episode 9 features a heartbreaking cameo appearance from original Ellie actress, Ashley Johnson.

Liane Hentscher/HBO

At no point in The Last of Us Episode 9 does anyone actually explain why Ellie is immune to the show’s central infection. However, the events depicted in the prologue of “Look for the Light” suggest that she has been living with the Cordyceps ever since her brief exposure to it when she was born. According to Marlene, Ellie hasn’t been turned by any of the other bites she’s received since then because the virus that has grown inside of her throughout her life “makes normal Cordyceps think that she’s Cordyceps.”

That means that, while Anna failed to sever her physical ties to Ellie before she was bitten, she still managed to cut her umbilical cord before her daughter was exposed to the Cordyceps for too long. Taking all of that into account, it’s easy to see why it’d be near-impossible for anyone to successfully replicate the process that resulted in Ellie’s immunity. Anna, after all, operated purely on instinct in the opening scene of The Last of Us Episode 9.

There is, consequently, no way of knowing exactly how much time would be too short or too long for a child to be exposed to the Cordyceps in the same way that Ellie was and end up immune rather than infected. Using Ellie’s immunity seems, therefore, like the only logical way anyone could mass-produce a cure for The Last of Us’ central outbreak — a fact that only adds even more weight to Joel’s actions in “Look for the Light.”

After the events of The Last of Us Episode 9, Ellie (Bella Ramsey) has a dark and complicated future ahead of her.

Liane Hentscher/HBO

The Inverse Analysis — In the same episode where the origin of Ellie’s immunity is revealed, Joel also makes the decision to save her life and single-handedly destroy the Fireflies’ plan to use her immunity to reproduce a cure. In doing so, Joel has effectively set the stage for the events of The Last of Us Part II, which lets the wasted potential of Ellie’s immunity haunt its story like an ever-present ghost.

For now, though, longtime Last of Us fans can sit back and enjoy the fact that, after waiting 10 years for an explanation, they finally know the truth about Ellie’s immunity.

The Last of Us Season 1 is streaming now on HBO Max.

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