In Stranger Things season 1, the group finds Will Byers alive in the Upside Down at the end of episode 8, “Chapter Eight: The Upside Down.”

When Will Disappears

  • Will vanishes on the night of November 6, 1983, while biking home after playing Dungeons & Dragons with his friends.
  • He crashes his bike, runs to his house’s shed, and then mysteriously disappears as the light flares and the Demogorgon drags him into the Upside Down.

When They Realize He’s Missing

  • Joyce and Jonathan notice he is gone the next morning and report his disappearance to Chief Hopper, starting the town‑wide search.
  • The official investigation, kids’ search, and Eleven’s arrival all unfold across episodes 1–3 while Will is trapped in the Upside Down.

When They Find Will

  • Hopper and Joyce track Will to the Upside Down version of Hawkins through the gate at Hawkins Lab in episode 8.
  • They find him unconscious with a tendril down his throat, cut him free, and resuscitate him, bringing him back to the real world by the end of the season 1 finale.

TL;DR: Will disappears in season 1, episode 1 and is finally found and rescued from the Upside Down in season 1, episode 8. Information gathered from public forums or data available on the internet and portrayed here.