Black Mirror Season 6 is here at long last, and there's a lot of hidden elements to unpack — including the return of a long-running Black Mirror joke about HBO.

Episode 1, "Joan Is Awful", a very meta (even by Black Mirror standards) story about a woman whose life is dramatised on a streaming website in almost real time, has plenty of Easter eggs to break down just on its own — and hidden among the episode's big twists and the exploration of AI in relation to creativity is a dig at one of Netflix's rival streaming services.

And it's all thanks to the return of Black Mirror's ever-present fake TV show, Sea of Tranquility.

What's the HBO reference in "Joan Is Awful"?

Before Joan (Annie Murphy) is plunged into her own meta-nightmare in "Joan Is Awful", there's a moment when she and her fiancé Krish (Avi Nash) are scrolling through "Streamberry" (the episode's fictional version of Netflix) to find something to watch.

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Krish suggests the show Sea of Tranquility, and the following description appears onscreen:

"HBO's seminal sci-fi western returns for a climactic tenth series. Follow our intrepid crew as they manifest destiny far beyond safety of the Inner Worlds."

Joan's response? "Eric said it blows."

Ouch.

This isn't just a one-off cheeky jab, though — Sea of Tranquility has actually been popping up in Black Mirror for years.

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Where has Sea of Tranquility appeared in Black Mirror?

The fictional show Sea of Tranquility actually appears in Black Mirror's first ever episode, "The National Anthem", where it's mentioned in passing as an "HBO moon western".

The apparent popularity of the fake series is made clearer in the Season 3 episode "Nosedive", when protagonist Lacie (Bryce Dallas Howard) hitches a ride to "Tranquility Con" with a group of the show's obsessive fans. Meanwhile, news updates in the Season 5 episodes "Smithereens" and "Rachel, Jack and Ashley Too" allude to a Sea of Tranquility reboot.

And of course, there's also another reference to the show in Season 6, in episode 4, "Mazey Day." Paparazza Bo (Zazie Beetz) takes non-consensual photos of former Sea of Tranquility actor Justin Camley and his lover, an action which ends in tragedy.

Is this reference to a "climactic tenth series" the last we'll hear of Sea of Tranquility? Given how much Black Mirror co-creator Charlie Brooker enjoys in-jokes, we're going to guess no.

How to watch: Black Mirror Season 6 is now streaming on Netflix.