What Stalks the Deep

Alex Easton receives an urgent summons from Dr. James Denton, an American physician who assisted ka^[a] during the events of What Moves the Dead. Denton's cousin Oscar has vanished while exploring an abandoned coal mine. Oscar's most recent letters stated that a mysterious red light had appeared inside the cave and that he planned to investigate. Oscar has now vanished, and a cryptic telegraph has warned Denton away from Hollow Elk Mine.

Easton and Angus travel to West Virginia. They are accompanied by Denton and his lover, chemist John Ingold. They first encounter Oscar's companion Roger, an incompetent drunkard who provides no helpful information. Easton's group begins exploring the mine. As the exploration continues, they hear unexplained noises, see a red light, and eventually see a glowing humanoid figure.

Outside the mine, villagers summon Dr. Denton to the scene of a supposed bear attack. While Denton treats the survivors, Easton and Ingold examine the corpse of a victim. The body has been completely eviscerated; they suspect foul play. Easton's group returns to the mine, where they find a chamber covered in a nacre-like substance. Soon afterwards, Easton's group finds a humanoid creature in the cave.

The creature calls itself Fragment. Fragment was once part of a hive mind and has become separated from the larger body, which lies beneath the nacre of the cave. The hive mind, which calls itself the “wholeness”, wishes only to be left alone. It regularly sends sentries into the outside world to monitor for human incursions. Fragment has recently been awoken, as the previous sentry has gone missing.

Easton's horse is attacked by a bear-like creature. The next day, Roger visits the mine, feeling guilty for abandoning Oscar. Easton is attacked by Roger's dog Thunder. The dog reveals itself to be the missing Sentry, the culprit behind the recent attacks. Sentry has developed a strong sense of individualism and does not want to rejoin the wholeness.

Fragment and Angus kill Sentry. Denton tries to kill Fragment, but Easton and Ingold convince him that this would be morally wrong. Easton realizes that another piece of Sentry has survived. This creature pursues ka through the mine. Easton uses flammable gases to set off an explosion, killing the remaining part of Sentry.

Easton's companions drill through the nacre, allowing Fragment to rejoin the wholeness. Denton hires Roger to guard the mine, keeping out human intruders. The wholeness also plans to keep guard for humans, ensuring to send multiple sentries in the future to ensure that none will become like Sentry.