The story is told from the perspective of a scientist called Bassett, who is on an expedition in the jungle of Guadalcanal to collect butterflies. The "Red One" of the title refers to a giant red sphere, of apparently extraterrestrial origin, that the headhunting natives worship as their god and to which they perform human sacrifices. Bassett becomes obsessed with the Red One and in the end is sacrificed himself.

The story's theme was suggested to London by his friend George Sterling: a message is sent from an alien civilization but is lost in the wilderness. There are parallels to Joseph Conrad's short novel Heart of Darkness.