The entire narrative of "Stranger Things" is driven by the consequences of experiments at Hawkins National Laboratory. In Season 1 (1983), a young girl with psychokinetic powers, Eleven, opens a gate to an alternate dimension, the Upside Down. A monster, dubbed the Demogorgon, emerges and abducts Will Byers. His friends Mike, Lucas, and Dustin find Eleven and, with the help of Will's mother Joyce and Police Chief Jim Hopper, they rescue Will. Eleven sacrifices herself to destroy the Demogorgon but is revealed to be alive, trapped in the Upside Down.
In Season 2 (1984), Will is suffering from visions of the Upside Down, revealing he is possessed by a massive entity called the Mind Flayer. Hopper secretly raises Eleven. A new gate has created a network of tunnels under Hawkins. Ultimately, Joyce and Jonathan exorcise the Mind Flayer's influence from Will by burning him, while Eleven, having reunited with her friends, uses her powers to close the gate, trapping the Mind Flayer in its own dimension.
Season 3 (1985) sees a secret Soviet lab beneath the new Starcourt Mall attempting to reopen the gate. This allows the Mind Flayer to possess people in Hawkins, including Max's stepbrother, Billy, using them to form a grotesque, giant monster of melted flesh. Billy sacrifices himself to save Eleven, and Hopper is believed to have died in the explosion as Joyce closes the gate. Following this, the Byers family, along with Eleven, moves away from Hawkins.
Season 4 (1986) reveals Hopper is alive in a Russian prison. A new, powerful being from the Upside Down, named Vecna, begins murdering Hawkins teenagers by preying on their trauma. A major twist reveals Vecna is Henry Creel, Dr. Brenner's first psychic subject (Number One), whom Eleven defeated as a child and banished to the Upside Down, where he was transformed. Eleven works to regain her powers to fight him. Despite their coordinated attack, Vecna succeeds in killing Max (though Eleven restarts her heart, leaving her in a coma) and opening four gates, which merge and begin consuming Hawkins, setting the stage for a final, apocalyptic confrontation.