Bête Noireis a long way from the bleakest episode of Black MirrorSeason 7,swallow tube sex video but its twist — or at least the sci-fi element of the episode — is probably one of the trickiest to get your head around.

The bulk of the episode plays out like a drama/mystery, only for the last 10 minutes to go impressively off the rails. So what happens at the ending of Bête Noire, and how does the device that Verity (Rosy McEwen) uses to alter reality actually work? We've done our best to break it down below.

What's Bête Noireabout?

Maria (Siena Kelly) is cheerfully excelling at her confectionary company job when her world is (literally) rattled by the arrival of former schoolmate Verity. There seems to be some tension between the two of them, with Maria trying to sabotage her getting the job and telling colleagues there were rumours about her at school. The problem? Maria herself was involved in spreading said rumours, making Verity's life a misery in the process.

While Verity quickly settles in at her new workplace, Maria gets increasingly confused. First she finds herself misremembering the names of places, and then she sends an email to Verity in which she swears she wrote one thing, but is proven to have written something else. Eventually she comes to believe that Verity is responsible for her mistakes, and that she's somehow able to change reality without anyone else knowing.

What happens at the end of Bête Noire?

After getting fired from her job, Maria follows Verity back to an impressively massive house that has a room filled with computers on the ground floor. She steals Verity's necklace, which she believes is the device she's using to change reality, and then confronts her former schoolmate. Verity, however, is not worried.

"It's just a remote," she says. "It connects to the quantum compiler downstairs. That'swhat changes reality."

Maria continues hopelessly brandishing her useless necklace/remote until Verity gives a more detailed – albeit equally confusing – explanation.

"Technically, it's not really changing anything; it just retunes our corporeal frequency to one of the parallel realities where whatever I've said has always been true," she says. "There's infinite timelines, so I just pick the one where you're the only one that knows what's going on. So you feel really... special."

In simple terms, Verity is saying that there are an infinite number of universes, where every imaginable combination of possibilities has taken place. Her remote allows her to communicate with the quantum compiler she's built, dictating the reality shewants. The compiler then places her in a parallel universe where what she's said is true, and Maria is the only person that's aware things have changed.

Still confused? So is Maria. But as Verity puts it: "I don't care if you understand it. I'm doing it to hurt you."

Why does Maria kill Verity?

Correctly ascertaining that she's completely powerless and that Verity's quantum compiler makes her close to a god, Maria does the only thing she can: She shoots Verity in the head, and then uses her remote to tell the quantum compiler that she's the new boss.

• 'Black Mirror' Season 7: 'Common People's very bleak ending, explained
• 'Black Mirror' Season 7: 'Hotel Reverie's ending isn't as romantic as you think it is
• 'Black Mirror' pulls 'Plaything' inspiration from notorious true crime case
• 'Black Mirror': 'Eulogy' episode explained
• Every 'Black Mirror' episode ever, ranked by overall dread

"The pendant works for me, the pendant works for me!" Maria yells into it, before quickly issuing another instruction to get the police to stand down. "She shot herself. It wasn't me. You saw the whole thing."

Against all odds, Maria's plan works. She finishes the episode in the same position of power that Verity had previously, making herself the new "Empress of the Universe." The last shot shows her standing on a plinth on what looks like an alien planet, surrounded by loyal subjects calling out "Hail, Maria!" in unison.

So... a happy ending?

Black MirrorSeason 7 is streaming now on Netflix.