The second episode of HBO Max's "Stuart Fails to Save the Universe" includes a surprise cameo from Kaley Cuoco, reprising her role as Penny in an alternate form. The episode, now streaming, is packed with cameos and guest stars, with Cuoco's appearance saved for near the end. It also brings back Zack, a recurring character from "The Big Bang Theory," as teased in the show's opening credits. The episode continues from the first, where Denise (Lauren Lapkus) became separated from Stuart (Kevin Sussman), Bert (Brian Posehn), and Kripke (John Ross Bowie) during a transportation mishap. Stuart makes a bold move as his relationship with Denise develops, while the fate of Gary remains uncertain. The show's ever-changing main title sequence also played a role in hinting at the episode's surprises.