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The Last of Usepisode 3, "Long Long Time," chronicling the love story of Bill (Nick Offerman) and Frank (Murray Bartlett), was an emotionally draining experience, with so many moving moments and hidden details that we almost missed one important throwback.

The thing is, though, episode 3 didn't just serve as a beautiful, contained story that shed more light on the different people surviving in a post-Cordyceps world — it also wrapped up a mystery from episode 1.

What happens in episode 1?

Towards the end of the show's first episode, just after Joel (Pedro Pascal) and Tess (Anna Torv) take charge of Ellie (Bella Ramsey) in the Boston quarantine zone, they leave her in their apartment while they discuss what to do in the hallway outside. Ellie overhears them say the names "Bill and Frank" before discovering a radio with a large book of songs sitting next to it. Tucked inside is a sheet of paper with "B/F" written at the top and a brief code scribbled below it that suggests different decades of music mean specific things. '60s is "nothing in", '70s is "new stock", and '80s has a red "X" next to it.

The minute Joel comes back into the room, Ellie starts interrogating him.

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"So, who's Bill and Frank? The radio's a smuggling code, right? '60s songs, we don't have anything new, '70s they've got new stuff — what's '80s?"

Later, after Joel has napped, Ellie tells him, "Oh, the radio came on while you were sleeping. It kept saying like, wake me up before you go, go.'"

"Wake Me Up Before You Go-Go" is a Wham! song from 1984. Sure enough, Joel's reaction — a muttered "shit" — suggests this is bad.

"Gotcha," Ellie grins. "'80s means trouble. Code broken."

Later, as Joel, Tess and Ellie make their way through the crumbling outskirts of Boston, we see a shot of their empty apartment as the radio plays Depeche Mode's 1987 song "Never Let Me Down Again". Trouble, it would appear, is afoot. But it's not until the end of episode 3 that we find out exactly what the code means.

What happens in episode 3?

In episode 3, we're finally introduced to the mysterious Bill (Offerman) and Frank (Bartlett), a gay couple who live alone in a fortified town a little way outside Boston. Joel and Ellie arrive too late, though — by the time they get there, Bill and Frank are both dead, having each taken a fatal overdose in response to Frank's terminal illness.

And sure enough, while exploring Bill's extremely well-equipped basement, they come across a radio.

"Why was the music on?" asks Ellie as Joel taps a button to stop it playing.

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"If you didn't reset the countdown every few weeks, this playlist would run over the radio," he replies.

Ellie looks at the screen and confirms what we're already thinking: "'80s."

Joel doesn't respond directly, but it's enough to confirm exactly why he muttered "shit" in episode 1 when Ellie tricked him into thinking a Wham! song had played. The radio code wasn't exactly "trouble" for '80s, like she guessed — it meant that something had happened to Bill and Frank to stop them from resetting the timer.

For someone as careful as Bill, Joel would have known that likely meant he was captured or dead. And that Depeche Mode song we heard at the end of episode 1? That meant Bill and Frank had likely already been dead for several weeks when Joel, Tess, and Ellie left Boston.

And it's not the only important use of music in the episode either.

The Last of Usis now streaming on HBO Maxwith new episodes airing weekly on Sunday nights on HBO.