HBO Max’s debut season of “Stuart Fails to Save the Universe” has come to a close, delivering a chaotic multiverse adventure for “The Big Bang Theory” fans. Created by Bill Prady, Zak Penn, and Chuck Lorre, the series puts Kevin Sussman back in the role of Stuart Bloom, the beleaguered comic book shop owner who gradually became a regular on the original sitcom. He is joined by Lauren Lapkus as Denise, John Ross Bowie as Barry Kripke, and Brian Posehn as Bert Kibbler—three familiar supporting players now at the center of the spinoff.

The premise of “Stuart Fails to Save the Universe” is as sprawling as its title suggests. After accidentally damaging a device created by Sheldon Cooper, Howard Wolowitz, and Leonard Hofstadter—played by Jim Parsons, Simon Helberg, and Johnny Galecki—Stuart is forced to move across alternate dimensions to stop a looming “multiverse Armageddon.” Denise, Barry, and Bert join him, encountering alternate versions of themselves along the way. That setup also opens the door for a long list of “The Big Bang Theory” cameos. Here is every returning character featured in Season 1.

Technically, one of the earliest familiar faces is not a “The Big Bang Theory” character at all. Penn and Teller appear near the start of Season 1, desperately performing a magic act in one of the show’s bleak, post-apocalyptic realities. Penn Jillette never appeared on the original sitcom, although he has built an extensive acting résumé, including a strange recent turn in the 2025 film “Marty Supreme.”

Teller does have a connection to the original series, however. He played Larry Fowler, Amy Farrah Fowler’s perpetually frightened, silent father, while Kathy Bates portrayed Amy’s intimidating mother. In the spinoff, Teller does not reprise Larry. Instead, he appears as himself, attempting to earn a little money in a devastated world.

Raj Koothrappali, played by Kunal Nayyar throughout “The Big Bang Theory,” is the first unmistakable returning character. In the opening universe, a post-apocalyptic wasteland, Raj is imprisoned by Barry Kripke, who has become a powerful warlord. Barry ultimately kills Raj while fighting over the multiverse-traveling device—but that is not Raj’s only appearance.

Raj returns in the sixth episode, “Spoiler: Corn Is Delicious,” set in a world where humans behave like cows and survive by eating grass. There, he stands on a street corner shouting that the multiverse is collapsing and no one can stop it. His various appearances are hardly happy ones, but seeing Nayyar back in the role is still a treat—even if Raj remains single in every reality.

Christine Baranski’s appearance falls into another technical category. On “The Big Bang Theory,” she made recurring appearances as Dr. Beverly Hofstadter, Leonard’s cold, exacting mother. Beverly is a neuroscientist and psychiatrist who frequently used her son’s life as material for her books. Her return in “Stuart Fails to Save the Universe” is somewhat more direct.

Baranski appears as Christine Baranski. In one reality, Stuart and his companions discover that electronic implants in their necks force them to remain relentlessly pleasant. When Barry, Stuart, and Bert are sent to a reformatory designed to turn them into cheerful citizens, they watch an instructional film hosted by Baranski, who introduces herself as an acclaimed actress. The claim is accurate: as of this writing, she has won one Emmy and several Tony Awards.

Brian Thomas Smith also returns as Zack Johnson, Penny’s former boyfriend from “The Big Bang Theory.” In the same dystopian universe featured in Baranski’s video, people are punished with electric shocks whenever an AI-controlled neurochip detects rudeness or emotional behavior. Zack serves as Penny’s loyal assistant in the resistance movement.

Zack helps Penny and the resistance disable the chips, but his role is mostly a brief supporting appearance. There is no major twist attached to him; he is simply back in the “Big Bang Theory” universe and ready to help.

Kaley Cuoco returns as Penny Hofstadter, one of the original series’ most recognizable characters. Penny was “The Big Bang Theory’s” only central female character until Season 4, when Mayim Bialik and Melissa Rauch joined the cast. In this alternate reality, Penny is a senior resistance operative working to escape the spread of the emotion-monitoring AI technology.

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Penny appears again in other dimensions, including Episode 8, “Spoiler: We’re as Confused as You Are.” In that world, a rigid military government court-martials the central group for treason and sentences them to death by firing squad. Penny is the one who reports them, yet her curiosity about the damaged device ultimately leads to her being transported—by mistake—to a universe inhabited entirely by centaurs. Surprisingly, she does not seem especially upset. The gag also nods to Cuoco’s real-life love of horses and her skill as a rider.

Melissa Rauch’s return as Bernadette Rostenkowski-Wolowitz is one of the season’s biggest comedic highlights. Bernadette, who married Howard Wolowitz in the Season 5 finale of “The Big Bang Theory,” appears in the third episode, “Spoiler: Bert Is Magic.” While Bert discovers that he has become a godlike magician, Bernadette is revealed to be a powerful witch working as a freelance spellcaster. She tells the group that she needs rare ingredients—including the blood of an innocent—to restore her abilities, then uses magic to repair the multiverse device.

The group ignores a warning from another sorcerer and obtains the blood Bernadette needs. She then transforms into a gigantic demonic version of herself, stomping through Pasadena and killing roughly half the city. Turning the petite Rauch into a monster of Godzilla-like proportions is one of the episode’s sharpest visual jokes. The sorcerer who tries to warn everyone about Bernadette is another familiar face.

Wil Wheaton reprises his “The Big Bang Theory” role as Wil Wheaton, the actor who became one of Sheldon Cooper’s favorite enemies. In the original sitcom, Wheaton played an exaggerated version of himself with a mischievous streak. His appearance in “Stuart Fails to Save the Universe” is brief, but he correctly identifies Bernadette as a demon and then engages Bert in a magical duel. It is a compact but satisfying callback for longtime viewers.

Mayim Bialik’s Amy Farrah Fowler appears after Bernadette and Penny, making her return feel inevitable—but the spinoff gives her a particularly inventive role. At the end of “Spoiler: Bert Is Magic,” Stuart wakes up in a universe where he is a patient in a psychiatric institution. His doctor is Amy, who understandably doubts his claims about alternate realities. When Stuart learns that he is married to Denise and has children with her, Amy allows him to return home and recover.

Amy appears once more in the penultimate episode, “Spoiler: We Couldn’t Get Green Lantern,” as the girlfriend of Jay Garrick, the Golden Age Flash played by Andy Ridings. Both of Bialik’s appearances are small, but the series finds clever ways to bring Amy into its increasingly strange alternate universes.

Neil deGrasse Tyson is another technically unconventional cameo, since he has always played himself in the “Big Bang Theory” world. Still, his return is too memorable to overlook. The real-life astrophysicist appears briefly in a universe where scientific knowledge is treated as sorcery—and where suspected practitioners are burned at the stake.

Riki Lindhome returns as Ramona Nowitzki, the only woman besides Amy Farrah Fowler to show a romantic interest in Sheldon Cooper. Her role is very different this time around. Rather than a scientist pursuing Sheldon, Ramona works as a receptionist at Caltech, the institution that serves as the main workplace in “The Big Bang Theory.” She appears several times, interacting with Stuart, Denise, Barry, and Bert.

Joshua Malina also makes a quick appearance as Caltech president Siebert, the administrator who tolerated Sheldon, Leonard, Howard, and Raj despite their frequent annoyances. Malina played Siebert throughout “The Big Bang Theory” and briefly returns in Episode 8, “Spoiler: We’re as Confused as You Are,” this time as a military lawyer. The part may also be a playful reference to his “Scandal” character, attorney David Rosen.

In the same episode where we see President Siebert as a lawyer, we also see Richard Williams, the Air Force officer who shows up in Season 10 of “The Big Bang Theory” and asks the guys to work on a missile guidance system. In “Stuart Fails to Save the Universe,” the Colonel appears as the head of a military force that bullies Stuart, Denise, Barry, and Bert into joining.

Season 1 of “Stuart Fails to Save the Universe” is streaming in its entirety on HBO Max, and if you like that series, here are five other shows you should check out next.