Henry Townshend is trapped in his flat, room 302 of South Ashfield Heights. After three days, a mysterious hole appears. He crawls in and finds himself in a twisted world of monsters and evil. As it turns out, the room used to be occupied by a child, Walter Sullivan, who’s been abandoned by his parents. Walter thinks the room itself is his mother (obviously), and that by murdering 21 people - an ancient ritual known as the 21 sacraments - he can awaken her. Along with his neighbor Eileen, Henry tries to stop him. But as Walter himself is the eleventh victim (he used what he calls the Ritual of Assumption to avoid death and become immortal) Henry has to make him mortal to finish him.
What it all means
The Room wasn’t originally intended to be part of the Silent Hill series, and so links to previous games are slightly clumsy. Walter was mentioned in a document in SH2, and Henry visits areas around Silent Hill several times, including the edge of Toluca Lake. There are also links between the cult that Walter was part of and the cult that tried to resurrect the god in the first game - they were in opposition to each other. A fan theory is that the darkness enveloping Silent Hill is extending beyond the town. Ashfield, where the game is set, is “just a few hours” from Silent Hill.