At the same time, Will. disappeared because he entered another parallel dimension caused by the One or Henry Creel. Will was cornered and felt weak, even though he had now become the host of the symbiote.

And you, Eleven's sister, you both escaped from the torturous lab. It's been a few days and you just found out from Joyce Byers that her son is missing. His name is Will Byers. You and Eleven only heard the conversation between Joyce, Jim Hopper who is a sheriff, they both argued, you saw he brother Will who looked frustrated.