S4E9 · Chapter Nine: The Piggyback

Complete story — includes the ending

The Hawkins group puts their plan into action: Max, Lucas, and Erica go to the Creel House while Steve, Nancy, and Robin go to its Upside Down counterpart to attack Vecna, with the bats being lured by Dustin and Eddie. Eddie sacrifices himself in the process. Eleven's group creates an isolation tank so she can enter Max's mind and fight Vecna. However, Vecna overwhelms her and possesses Max, revealing to Eleven that he has controlled the Upside Down since she sent him there. Mike professes his love for Eleven, giving her the strength to break Vecna's control over Max, but after Jason interferes, Max dies from her injuries. Hopper, Joyce, and Murray re-enter the prison and kill the remaining Demogorgons, weakening Vecna. Steve, Robin, and Nancy set fire to Vecna's physical form and shoot him, seemingly killing him. Eleven uses her powers to revive Max, but Max's brief death allows Vecna's portals to open and pass through Hawkins, killing Jason. Two days later, the town recovers from an "earthquake." Everyone reunites, while Max remains comatose. Will senses that Vecna is still alive, and the Upside Down begins to invade Hawkins.