Warning: Spoilers ahead! Do not proceed unless you’ve watched the fourth episode of “Stuart Fails to Save the Universe.”

She came in with a big bang. 

In the fourth episode of “The Big Bang Theory” spinoff, “Stuart Fails To Save The Universe,” original “Big Bang Theory” star Mayim Bialik makes a surprise return. 

“It was so much fun, it was like an awesome reunion,” series star Kevin Sussman told The Post. 

Kevin Sussman in “Stuart Fails to Save the Universe.” Colin Remas Brown/HBO MaxMayim Bialik (left) and Jim Parsons in “The Big Bang Theory” in 2016. ©CBS/Courtesy Everett Collection

In the episode, Stuart (Sussman) lands in an alternate dimension where he’s being held in a psychiatric hospital with Bialik playing his therapist.

She’s skeptical of his accounts of traveling through alternate realities, including a world where he knew her as Amy, her neuroscientist character from “The Big Bang Theory.” 

“It’s funny, because when we were working on ‘Big Bang,’ I had a very specific relationship with all of the cast members,” Sussman told The Post. 

Lauren Lapkus and Kevin Sussman in “Stuart Fails to Save the Universe.” HBO MAX“The Big Bang Theory” stars Kaley Cuoco, Johnny Galecki, Melissa Rauch, Kunal Nayyapose and Simon Helberg, Steven Molaro, Jim Parsons, Mayim Bialik, Chuck Lorre and Bill Prady at the People’s Choice Awards on January 6, 2016. Smallz & Raskind

He noted that he used to “talk politics” with “The Big Bang Theory” star Jim Parsons, and he discussed acting with co-star Simon Helberg. 

“And Mayim was basically my on-set therapist,” he said. “That’s just a coincidence. I don’t think [series creators Chuck Lorre and Bill Prady] knew that.”

He noted that because of their behind-the-scenes relationship, it was “very fitting” for Bialik to appear on the show as Stuart’s temporary therapist. 

Jim Parsons, left, and Mayim Bialik in “The Big Bang Theory.” ©CBS/Courtesy Everett CollectionMayim Bialik, left, and Jim Parsons in “The Big Bang Theory.” ©CBS/Courtesy Everett Collection

“The Big Bang Theory” aired for nearly 300 episodes on CBS, from 2007 to 2019, drawing 20 million weekly viewers. 

The hit sitcom followed a group of nerdy friends, including Sheldon (Jim Parsons) and Amy (Bialik). Comic book store owner Stuart Bloom was a side character. 

In the spinoff, “Stuart Fails to Save the Universe,” he takes center stage, journeying into a different universe each episode. Sometimes, familiar faces from the original show pop up, playing different characters. 

Kevin Sussman in “Stuart Fails to Save the Universe.” HBO MAXLauren Lapkus and Kevin Sussman in “Stuart Fails to Save the Universe.” Colin Remas Brown/HBO Max

Sussman added that he and Bialik had “kept in touch” after “The Big Bang Theory” ended. During the Covid-19 pandemic, they participated in a Dungeons & Dragons game over Zoom, along with several of the show’s writers, and fellow actor Brian Posehn.

“But, I hadn’t worked with her in years – and she is so freaking funny in this,” he said.

Fellow “The Big Bang Theory” alum Kaley Cuoco already made an appearance in the second episode of “Stuart.” 

Kaley Cuoco in “Stuart Fails to Save the Universe.” HBO MAXBrian Posehn, Kevin Sussman, Kaley Cuoco and John Ross Bowie in “Stuart Fails to Save the Universe.” HBO MAX

“I hadn’t seen Kaley since ‘Big Bang’ ended,” said Sussman. 

The cast was together for “so long – and, we were all so close on the show,” he said, noting that after it ended, “everyone has their careers that they go off and do.” 

When Cuoco and Bialik appeared on their respective episodes of “Stuart Fails to Save the Universe,” Sussman said it was “amazing” how “we just fell right back into the same friendship that we had on set. It was just delightful.”

John Ross Bowie, Lauren Lapkus, Kevin Sussman and Brian Posehn in “Stuart Fails to Save the Universe.” HBO MAX“The Big Bang Theory” cast Kunal Nayyar, Mayim Bialik, Simon Helberg, Johnny Galecki, Melissa Rauch, Kaley Cuoco and Jim Parsons at the People’s Choice Awards on Jan. 6, 2016. Frazer Harrison

Could Parsons be next? 

“I think he generally wanted to move on and do other stuff,” Sussman told The Post. 

“But I could see him maybe doing an episode, I’ll leave it up to him. I know that whatever he did, he would knock it out of the park.” 

“Stuart Fails to Save the Universe” is now streaming on HBO Max, with new episodes out Thursdays at 9 p.m. ET.