You've likely waited over six months for a fresh hit of The Last of Us's story - so the last thing I'm going to do, with less than a week to go, is spoil for you what makes Left Behind, a prequel of sorts that explores some of Ellie's backstory, so special. I'll share a few numbers with you, though: Left Behind takes some two hours from beginning to end, and in that time I welled up once, while thinking to myself that Naughty Dog has done it again a couple times more. It's an expansion as brave and thoughtful as the original, retaining the quality of Joel and Ellie's story and enhancing it in subtle, moving ways.

"We were worried that we'd somehow diminish the experience of The Last of Us," Neil Druckmann, writer and creative director on The Last of Us and its Left Behind DLC, says when I tell him that, in my opinion at least, it's a success. "So that's great to hear."

Seeing as we can't exactly discuss what it is about the story-beats and set-pieces that works so well right now - we'll be looking to do that soon after Left Behind's release - the best place to start is exploring what this DLC isn't. Naughty Dog was open about working on another chapter within the world of The Last of Us soon after the release of the original, but before the precise details were made clear there was some speculation that it would centre on Ish, a character sketched out through background detail as players made their way through The Suburbs. This was a possibility considered by Naughty Dog, but one it ended up opting out of.

It's only a couple of hours long, but a good number of surprises fit into the run-time.

"The only reason we discussed it is because we thought there was so much demand for it," explains Druckmann. "The thing with Ish, I think it's one of those things that's better left to the imagination - everyone has a different idea of what he looks like, how old he is or how he'd behave. It felt like we'd take something away from that by fleshing it out. Even in the DLC we actually had at one point a note that we wrote out that you'd find that continued Ish's story. It was a really moving note about him and his relationship with Susan, but it felt too much like fan service, too much of a coincidence that you'd find this note."

Class war Left Behind introduces a new weapon type - but we'll let you discover that for yourself - as well as a new flavour of combat, with the infected now able to clash against non-infected foes while you wait in the shadows. It's a fitting addition when you're playing as the physically weaker Ellie, and introduces a new layer of strategy to the combat. "It is something we wanted in the original game - we had some areas in Pittsburgh where we wanted to use it. We'd set them against each other and add more strategy to the combat. But it's one of those things where the game was so ambitious and so big, we kind of had to say... We had a prototype, and it was good but we didn't have enough time to polish it. And the story didn't require it, so it was one of those things."

The idea that made sense, though, had already been suggested in one of the main game's more powerful moments, as well as providing the foundation for American Dreams, a tie-in graphic novel penned by Druckmann and Faith Erin Hicks. It centred on the relationship between Ellie and Riley, a friendship played out against the backdrop of a quarantine zone. "We had a quick brainstorm of ideas, and pretty quickly none of them lasted more than a sentence or two," says Druckmann. "We'd keep coming back to this story. It felt like a story that could stand on its own and had its own arc, that fit with the themes of The Last of Us but more importantly, it gave new light on the main story. It fits all those criteria on what would make a story worth telling."

Left Behind's focus on an adolescent relationship takes it to some fascinating places, and makes it - in parts - a much more playful outing. It's set against an abandoned shopping mall, a standard backdrop for post-apocalyptic narratives most famously used in George Romero's original Dawn of the Dead, but seeing it through the half-bemused and half-delighted perspective of a pair of teenage girls gives it a fresh twist. "People bought this stuff? I just don't get it," Ellie says as she walks through aisle upon aisle of so much rotting tat, before later being taken aback by the spectacle of a carousel in full bloom.

Left Behind fills in a few gaps in the existing The Last of Us story, and fleshes out Ellie's story. It's an origin tale, of sorts.

"It was such a sweet idea," says Druckmann. "This is what teenagers do. They skip school and go to the mall, they goof around and have fun. What's the post-apocalyptic version of that? Because they live these harsh lives, it's really interesting to us to say what if we play with that, and with two teenagers goofing around and having fun, but then you still see the reality of that world. Where we can afterwards go home - maybe you'd get grounded if you skip school - for this there are much greater consequences if you screw up."

Riley and Ellie's relationship, as told across the two hours of Left Behind, ends up as convincing - and as emotionally powerful - as that between Ellie and Joel. It's a different dynamic, and a different type of bond, but again it's one forged through mechanics as much as it is through the perfectly pitched cut-scenes. "In the original story campaign, we wanted to have these ideas where, how do we push relationship building outside of combat," says Druckmann. "With this, we really wanted to stretch ourselves to go past our comfort zone - what else can we do to build relationships outside of combat? We felt like the best moments in the Last of Us were the ones where we had this really great contrast between the horrible things you'd witness then these really sweet moments like the giraffe sequence."

There are sweet moments throughout Left Behind, as well as tragic ones. Some of that tragedy won't come as a surprise to anyone who's played through the original to its conclusion, which makes Naughty Dog's decision to have Riley's story at the centre of its expansion all the more brave, and all the more strange. "Before we committed to this side story, we challenged ourselves and said, is this a story worth telling, because people already know how it ends," says Druckmann. "Is there anything new here, and is there any other insight I could gain from this? And it's not so much about how it ends. It's about this journey, and seeing the impact the relationship has with Ellie and how deep it went. It felt like a new insight into who Ellie was."

Not long to wait now to see how Ellie and Riley's story plays out, and how Naughty Dog draws a line under one of the best stories the last generation has had to offer. We'll have a full review, and a little more insight from Druckmann, later next week.