Mayim Bialik, known for her role as Amy Farrah Fowler in the long-running sitcom “The Big Bang Theory,” made a surprise appearance in the fourth episode of the HBO Max series “Stuart Fails to Save the Universe.” In the episode, titled “Stuart Fails to Save the Universe,” Bialik plays a therapist to Kevin Sussman's character, Stuart Bloom.

In the alternate dimension depicted in the episode, Stuart is institutionalized in a psychiatric hospital. Bialik's character is skeptical of his claims about traveling through various realities, including one where he knew her as Amy, his neuroscientist girlfriend from “The Big Bang Theory.”

Sussman, who stars as Stuart in the spinoff, described the reunion as “so much fun” and “like an awesome reunion.” He noted that Bialik's role as his therapist in the episode felt fitting due to their personal relationship on the set of “The Big Bang Theory.”

"And Mayim was basically my on-set therapist," Sussman told The Post. "That’s just a coincidence. I don’t think [series creators Chuck Lorre and Bill Prady] knew that."

“The Big Bang Theory” aired for 12 seasons on CBS from 2007 to 2019, attracting an average of 20 million viewers weekly at its peak. The show followed a group of academically gifted but socially awkward friends. Stuart Bloom, the owner of a comic book store, was a recurring character.

In “Stuart Fails to Save the Universe,” the focus shifts to Stuart as he embarks on episodic journeys through different universes. The series occasionally features familiar faces from the original show, portraying different characters.

Bialik and Sussman have remained in touch since “The Big Bang Theory” concluded. They even participated in a virtual Dungeons & Dragons game over Zoom during the COVID-19 pandemic, alongside other writers and actor Brian Posehn. Sussman commented on Bialik's performance, stating, “But, I hadn’t worked with her in years – and she is so freaking funny in this.”

Another “The Big Bang Theory” alum, Kaley Cuoco, who played Penny on the original series, previously appeared in the second episode of “Stuart Fails to Save the Universe.” Sussman shared that reconnecting with Cuoco for the show was a delightful experience, noting how easily they fell back into their established friendship.

While Jim Parsons, who played Sheldon Cooper, has generally focused on other projects since “The Big Bang Theory,” Sussman indicated that a potential cameo is not out of the question.

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

New episodes of “Stuart Fails to Save the Universe” are released on Thursdays at 9 p.m. ET on HBO Max.