The Black Door follows a mother surviving alone after a spiritual apocalypse where entities have infested humanity. After a year of searching for her son, Charlie, who vanished from their basement without a trace, she finally decodes his cryptic CB radio transmission. His coded message references his old rocket-patterned pajama shirt, which she realizes contains a blueprint for building a device to reach him. As she completes the machine, mysterious strangers calling themselves "Camp One-One-One" contact her through the CB and issue a chilling warning: whatever speaks through the radio is not her son. When they arrive at her house demanding she open the door, trapped between maternal conviction and mounting dread, she must decide whether to activate the device she's built.