Kit, Andy, and Steve turn up the heat in one of Season 3’s most electrifying and suspenseful episodes. In Chapter Four: The Sauna Test, the group begins to suspect that Billy may be under the control of something from the Upside Down, and devises a dangerous plan to prove it. Meanwhile, Joyce and Hopper’s investigation into the mysterious power outages takes them on a chaotic road trip to find answers, and Dustin, Steve, and Robin recruit Erica to help them spy on the Russians.