For their new assignment, Sapphire and Steel arrive at an abandoned railway station haunted by supernatural figures. Paranormal investigator George Tully has been observing the station for several weeks because he has an interest in speaking with the ghost of a young man often heard whistling in the hallways. Sapphire and Steel indulge Mr. Tully by allowing him to help their investigation but Steel warns him that the apparition is something far more dangerous than a ghost.
Sapphire and Steel discover the ghost is a young World War I soldier who died in battle. They begin to suspect the railway station might be a recruitment ground for the dead. The appearance of a second ghost and the discovery of a recording with the last words of three dying submariners all but confirm their suspicions. The investigation becomes even more difficult and dangerous when the ghost of a fighter pilot forces Steel to experience a deadly plane crash.
After Sapphire narrowly saves Steel from meeting the same fate as the doomed fighter pilot, Steel becomes determined to figure out why the ghosts are being recruited at the station. Since the ghosts have such strong emotions, Steel tries to anger the soldier into appearing by singing an old military marching song with Tully. Steel's tactic works a little too well and the soldier takes out his anger on Sapphire and Tully.
Steel continues to taunt the ghost of the solider but pauses when he finds Sapphire and Tully unconscious. A hallway in the railway station has become a re-creation of a submarine where three young men died from lack of oxygen. Sapphire and Tully nearly suffocate but Steel manages to carry them to safety before the three of them meet the same fate as the young submariners. Afterward, Sapphire and Steel agree to give Tully's methods a chance by holding a séance to find more information about the solider when he was alive.
With Sapphire as a medium, Tully and Steel are able to communicate with the ghost of a woman who was once the soldier's schoolteacher. As they prod the spirit with questions, she reveals that the force helping and controlling the ghosts is a terrible, living darkness. She also gives them the name of the soldier and the details of his untimely death. Satisfied with the information, Tully and Steel finish up the séance but an enveloping darkness creeps through the room and threatens to devour them all.
With Sapphire unconscious, Steel is unnerved by the appearance of her ghost on the railway platform with the other spirits. After Tully agrees to watch over Sapphire's body, Steel has a chat with her ghost to determine if she is real or a trick perpetrated by the darkness. The soldier is not through with trying to get rid of Steel, however, and snares him in barbwire. Meanwhile, the darkness makes a deal with Tully: he can escape the station unharmed if he leaves Sapphire and Steel while they are unconscious and vulnerable.
Sapphire and Steel awake to find they have been shunted forward in time twelve days. In that time, the darkness has gone and so have all the ghosts with it. A frightened Tully tries to convince Sapphire and Steel that he needs to leave the station for personal reasons, but they can guess he wants to leave because he made a deal with the darkness. While Tully's bargain failed, Sapphire reluctantly agrees to make contact with the darkness so that Steel can make his own deal with the enemy.
Sapphire succeeds in making contact with the darkness at a great personal risk. Steel negotiates with it by promising a powerful source of resentment in exchange for a return to present day. The solider finally reveals what it is the darkness has promised all the ghosts but Steel gives them a better offer. The darkness follows through with its part of the bargain and Sapphire and Steel must uphold their part as well, even if their actions damage time itself.