Short answers, pulled from the story.
Henry Townshend finds himself trapped inside his apartment with a locked door and no way to call for help. A hole appears in the bathroom wall that leads to a different world where he can save progress or store items.
Walter Sullivan is an undead serial killer who seeks to complete a murder ritual using twenty-one victims. He believes his biological mother resides within Henry's apartment after being abandoned there at birth.
Four possible endings occur depending on whether Eileen Galvin survives the final boss fight. One ending shows her dead on the radio while another depicts her planning to return to the building with flowers.
The game was released in Japan on June 17 and in North America on September 7. Team Silent developed the fourth installment in 2003 before its release.
Mark Z. Danielewski's novel House of Leaves published in 2000 inspired the non-Euclidean architecture of the apartment. Ryū Murakami's 1980 novel Coin Locker Babies provided the premise of an abandoned child seeking his mother.