The Demogorgon, also known as the Monster, was a predatory humanoid creature that entered Hawkins, Indiana in November 1983. The creature seemed to originate from the parallel dimension known as the Upside Down.[1] When Eleven, a child test subject from Hawkins National Laboratory with psychokinetic abilities, made transdimensional contact with the creature, a gate between dimensions opened at the lab. The creature then set foot in Hawkins, and began hunting, killing and kidnapping various local townspeople.

The friends and family of Will Byers, one of the Demogorgon's victims, slowly began to uncover the truth about the creature, with the help of the escaped Eleven. The creature was finally defeated in a showdown with Eleven at Hawkins Middle School. Eleven used her abilities to disintegrate the creature, but accidentally transported herself to the Upside Down in the process.