However, for die-hard fans, this wasn't new territory: we've previously seen Ellie's ill-fated coming-of-age romance depicted in the 2014 DLC add-on Left Behind.
https://www.youtube.com/watch?v=YdCzSZfPRGo
And that wasn't the only time we met Riley either - she was a main character in comic book spin-off American Dreams, which acts as a prequel of sorts to this week's instalment.
However, for more casual fans, this week's episode acted as an introduction to the character played by Storm Reid - but even within the original source material, she was a crucial part of Ellie's journey.
Introducing Riley
In the 2013 game, Riley is only mentioned in passing, as Ellie tells Joel that she lost someone close to her as they head towards Jackson. This wasn't explored in further detail until the later spin-offs came along.
In the comic book series American Dreams, we are first informed of an earlier excursion where Riley is still living at the military boarding school with Ellie in Boston.
That narrative sees them both break out overnight, stumbling into an active Firefly mission in the process.
DLC Left Behind then flashes forward and follows the same events as this week's episode. Now a Firefly, Riley returns to visit Ellie, encouraging her to break out of school and go on an ill-fated adventure at a nearby shopping mall.
It's not directly referenced in the TV adaptation, but these events take place roughly three weeks before Ellie meets Joel.
Ellie's First Love
Because we live in the age of the internet, you'll likely have encountered several tweets from supposed die-hard fans claiming that the show is "too woke".
This is despite Ellie being an established lesbian character in the games, with this crucial coming-of-age chapter of her life already having been told in a different medium.
Neil Druckmann, the creator of the games and co-creator of the HBO adaptation, has previously said that the relationship between the two girls was never initially intended to be romantic - but as he and comics illustrator Faith Erin Hicks were discussing their close friendship, they realised it made more sense that way.
Back in 2014, when the DLC was released, he told Wired: "When I did interviews with Faith for the comic book, we both made the mistake when we were describing Ellie and Riley for the first time – we said Ellie was attracted to Riley.
"We meant, I think, that she was just really drawn to her, that she really looked up to her.
"But then I thought in the back of my mind, 'what if there's something more than that?' It became this interesting dimension of their relationship that we could explore."
A Tragic Queer Romance
The decision to adapt the DLC story for this week's episode makes for an interesting contrast from the much acclaimed third episode, which brought the relationship between Bill and Frank to life for the first time in franchise history.
That bold decision subverted the source material, where we knew only that they ended on bad terms, instead depicting their personal utopia in the middle of a ruined America.
With the outbreak erupting in 2003, more than a decade before gay marriage was legalised in the US, it made for surprisingly poignant viewing: here were two men able to celebrate their love together for the entirety of their lives, away from a wider world that would have ostracised them.
It may have not been a peaceful existence but it's safe to say that they got to experience the conventional American married life that would have alluded them had the world carried on as normal.
https://www.youtube.com/watch?v=UlCEa_K3i44
The short-lived relationship between Ellie and Riley, on the other hand, is one mired in tragedy. Ellie's self-realisation of her sexual identity and feelings for Riley are cut short mere seconds after they share their first kiss, as an infected attacks them both.
It's an altercation that puts Ellie's character in a new light throughout the rest of the series. Think back to the end of episode three: she never meets Bill or Frank, only learning that they died together - further colouring in a lingering belief that relationships can only end in tragedy.
Does Riley Die?
One question the episode doesn't fully answer regards Riley's fate at the end, something which is copied over from the source material.
It's widely assumed that she would die - after all, Ellie is a miracle for being able to survive as normal after being infected - but it is never explicitly confirmed.
Going back to that Wired interview from 2014, Druckmann insisted that it was always his intention to leave that resolution as ambiguous.
He said: “There's a book by David Mamet where he talks about how you try to enter a scene as late as possible so that the viewer is engaged and trying to fill in the backstory. And you leave as early as possible, so if there are things you don't have to show, you don't show them.
"This story, this arc, was about how Ellie leaves her childhood behind and becomes an adult. There is another story to tell there about what happens afterwards, but in terms of this arc that was all we needed to tell and no more.
https://www.youtube.com/watch?v=MkZzyQt68iI
"You can fill it in yourself, or maybe one day I'll team up with Faith and tell the rest of the story.”
Maybe we'll get that ending filled in at some point during season two.
The Last Of Us airs Sunday nights at 9pm on HBO.
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