The story opens with the narrator recounting an eerie visit from his old friend, John Murken, an explorer and adventurer known for his fearless nature. However, Murken arrives at the narrator's apartment in a state of frantic terror, claiming he dares not sleep due to a recurrent nightmare that threatens his sanity and life. His appearance is haggard, his fingers bloody from a desperate attempt to stay awake.

Murken recounts a harrowing experience in India, where he and a Hindu guide sought a hidden treasure in a cavern. They were ambushed by the sons of Alam Singh's men, who killed the guide and bound Murken to the cave floor, placing a large cobra just out of his reach. The captors tied the cobra with a hide thong that gradually stretched as it was saturated by dripping water, bringing the deadly snake closer to Murken with each strike.

Murken endured hours of terror, watching the snake inch closer, anticipating his death by its venomous bite. Just as the snake was about to reach him, a group of tiger hunters stumbled upon the scene, killing the cobra and saving Murken in the nick of time. Despite his rescue, the trauma left a deep imprint on Murken's psyche.

After returning from India, Murken began experiencing vivid, recurring dreams of the same scenario. Each dream replayed the terrifying moments in the cave, with the snake progressively closer. He realized that the thong in his dreams was stretching faster each time, and he became convinced that if the snake struck him in his dream, he would die in reality.

Murken's narrative deeply unsettles the narrator, who tries to convince him that facing the dream might resolve his fears. Murken, resigned to his fate, finally agrees to sleep in the narrator's apartment. Despite his initial resistance, exhaustion overtakes him, and he falls asleep. The narrator, too, struggles to stay awake but eventually succumbs to sleep.

In the early morning hours, the narrator is awakened by Murken's blood-curdling scream as he yells about the lamp going out—a detail from his nightmare. When the narrator turns on the light, he finds Murken dead, his face twisted in horror, his right hand gripping his left wrist tightly. The narrator concludes that Murken's death was caused by the sheer terror of his dream, proving his belief that the nightmare could indeed claim his life.