"Three Skeleton Key" is about three men who work at a lighthouse on an island named Three Skeleton Key. It focuses on one incident where a ship full of rats crashes on the island, and the men must fight to stay alive.
The three men working at the lighthouse are the unnamed narrator, Le Gleo and Itchoua. They spot the abandoned ship as it approaches, and then see the rats as it gets closer. These rats are large and vicious, and the men surmise that the rats killed and ate the ship's crew.
When the ship crashes on the island, the men secure the lighthouse to keep the rats out. They spend many days and nights trapped, and although they have supplies, the next supply ship isn't scheduled to arrive for weeks. They're unable to signal for help, as they'd have to go outside.
The seal on a window gives out one night, and the men must fight off the horde of rats. They lock themselves in the lantern room. Without food or water, they decide to avoid lighting the lantern, which is usually only done if everyone in the lighthouse is dead.
Their plan eventually results in them being saved, after the coast guard helps kill the rats. However, the incident drives Le Gleo insane, and Itchoua dies from an infection. The narrator decides to continue working at the lighthouse.