In November 1983, he vanishes somewhere near "Mirkwood" after encountering a monster that escaped through a rift to the "Upside Down", an alternate dimension discovered by the Hawkins Laboratory scientists.

Will is harassed by local residents, who dub him "Zombie Boy" after apparently returning from the dead (due to a hoax orchestrated by the people from Hawkins Laboratory).

Dustin shows the group a slug-like animal he found and named D'Artagnan, and Will realizes he heard it during one of his visions.

Hopper is saved by the group, but scientists from Hawkins Lab set the tunnels on fire, causing Will to collapse and convulse in agony.

Will instructs the scientists to head to a specific spot he cannot see, but it is revealed that the Mind Flayer is controlling him and has led everyone into a trap.

Will reveals his connection, and the group theorize that Billy is possessed and trap him in a sauna to discover the truth.

At the end of the season, Will moves out of Hawkins with his mother, brother and Eleven after giving away his Dungeons & Dragons set to Erica Sinclair.

Eleven is arrested but soon intercepted by Dr. Owens, who takes her to a research facility in Nevada to participate in a project called "NINA" that is aimed at restoring her powers.

[12] In 2017, Schnapp was nominated for the category of Best Performance in a Digital TV Series or Film – Young Actor.

[13] Viewers had speculated that Will is gay and harbors feelings for his best friend Mike, which the series itself had not confirmed nor denied.

[14][15][16][17] Critics interpreted the final two episodes of the fourth season as subtextually acknowledging Will's sexuality, particularly during a scene where Will shows Mike his painting of him and his friends, claiming it to be a gift from Eleven, only to silently cry afterwards.

[20] Schnapp himself stated in July 2022 that he believes that it is "100% clear" and a "very real, obvious thing" that Will is gay and is in love with Mike.