Big Bang Theory's Penny makes surprise spin-off return looking very different from how fans remember

Kaley Cuoco made an unexpected return to The Big Bang Theory, seven years after the popular sitcom ended.

If the show ended several years ago then how did Cuoco make a return, I hear you ask? On the brand new spin-off Stuart Fails to Save the Universe, of course.

The new show, which streaming from July 24 exclusively on HBO Max, sees beloved characters from hugely successful series The Big Bang Theory face an apocalyptic dilemma.

Comic bookstore owner Stuart Bloom (played by Kevin Sussman) accidentally breaks a device built by Sheldon and Leonard.

His actions initiate a multiverse Armageddon and Stuart races against time to restore reality.

His girlfriend Denise (Lauren Lapkus), geologist friend Bert Kibbler (Brian Posehn) and quantum physicist Barry Kripke (John Ross Bowie) join Stuart on this madcap endeavour.

As Stuart and co repair the temporal damage, they encounter alternate-universe versions of characters from the main series.

But not all is as it seems in the new universe as the trio quickly realize they're wearing neck devices that shock them if they argue or act aggressively. They eventually find themselves in a a 30 month-long reeducation program designed to eliminate conflict.

Their woes are far from over then they're released, and they are abducted and taken to a random warehouse.

When their hoods are removed, they're greeted by Penny – but it isn't the waitress turned pharmaceutical rep we know and love from The Big Bang Theory. Instead it's a version of Penny that leads a resistance movement against an AI-controlled world alongside her ex-boyfriend Zack (played by Brian Thomas Smith).

To get the trio out of relentless positivity, Penny literally slaps them across the face.

Kaley Cuoco is back as a very different version of Penny (HBO Max)

Show-runner Chuck Lore has reflected on Cuoco coming back as a totally different Penny.

"I think at the end of the day, we handled some of those guest appearances beautifully in ways you don’t anticipate," Lorre told PEOPLE about Stuart Fails to Save the Universe. "Because we’re in an alternate universe, a different reality, Penny becomes someone else. She’s still Penny, but she has an entirely different background. She never worked at the Cheesecake Factory, never met Leonard. She’s a leader of a revolutionary guerrilla group, and she’s not averse to violence."

He added that he thought it was 'a wonderful thing to see Kaley Cuoco smacking the sh*t out of our hero'.

Lore echoed similar sentiments to Variety. He said about inviting some of the original Big Bang Theory actors to reprise their roles: "A big part of it was offering them a challenge to do something entirely different.

"As you’ve seen in Episode 2, Kaley’s not playing the server in the Cheesecake Factory. It’s still Penny, and now she’s a badass guerrilla leader.

"That was part of the design from the very beginning. In a multiverse environment, these characters could recur as themselves, but with an entirely different backstory."

Stuart Fails to Save the Universe drops new episodes on HBO Max each Thursday.