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This article contains plot details about an episode.
Addy's Basement exists in Addison Carver's dreams.
The basement exists is in Addy's subconscious as well as in the real world.
While exploring the Ruins, repeatedly trying to kill the unkillable zombie, Mack Thompson and Addy find a strange door. After opening the door and breaking through the boards blocking the entrance, Addy and Mack go down the steps into the basement, where Addy re-experiences the events of the first night of the outbreak: where she was forced to kill her mom and her brother Michael, since they were both zombified.