This is basically, What the story is about.
After the events of the Starcourt Disater, The Russians had found Billy's body, and kidnapped him.
It was assumed that he had died that day in Hawkins, but, he showed up months later bruised and bloody from russians experimenting on him and/or abusing him.
He showed up at 3am, on steve Harrington's front porch (because he was the closest from his dropping point), pleading to be let in.
