Gideon Drake, a 67-year-old retired operative, boards the Caledonian Sleeper train at Edinburgh Waverly. Despite his initial reluctance for the journey, a strange sense of unease settles over him as he observes a nervous man, an observant history teacher named Vivien, and the professional but weary train guard Gavin. A sudden storm and a peculiar disturbance on the train signal the start of an unusual journey.

As the storm intensifies, Gideon's instincts tell him something is wrong. After a violent jolt, the nervous man in the suit vanishes from the lounge car, and no one else seems to remember him. Gideon finds a cryptic note in an empty cabin: 'It didn't have to be this way,' confirming his suspicions of a deeper mystery.

Gideon discusses the unsettling events with Vivien, who also noticed the disappearance. Gavin, the train guard, reports communication issues with the crew. Another violent jolt occurs, and Gideon discovers a second identical note and a bloodstain in another empty cabin, indicating a more sinister turn of events.

Gideon, Vivien, and Gavin decide to keep the discoveries secret to avoid panic. They question Manuel, the dining car attendant, who mentions passengers looking at him as if they know him. They confront Gavin, who is dismissive. A loud clang and screech of brakes lead them to the empty engine room, where the driver is gone, and a third identical note is found on the console.

With the driver missing, the train is now uncontrolled. They search the train, finding a disheveled cabin with a burnt metal smell and a circular scorch mark, where a fourth note is found. Gideon glimpses a shadowy figure in the corridor. They then find themselves trapped in an empty, metallic car, which transforms into a strange, interrogation-like room with a blinking device that speaks the same cryptic message: 'It didn't have to be this way. You were warned.'

Plunged into darkness, Gideon, Vivien, and Gavin discover a glowing panel in the strange room. Activating it causes the room to shudder and transports them back to the corridor. Upon returning to the lounge car, the 'nervous man' is back, seemingly alive, but then he slumps forward, dead, with blood trickling from his mouth. Another note, 'It didn't have to be this way,' is found.

They discover the dead man is Edward Lane. Gideon finds a note in his pocket: 'They're watching. Don't trust anyone. It's already too late.' They discreetly move Edward's body. Gideon suspects someone on the train is involved. They witness more strange activity outside, deepening the mystery.

Tension rises among the remaining passengers. Gavin discovers another deceased passenger, Karen, in her cabin, with an identical note confirming the pattern of inexplicable deaths. Gideon confronts the passengers, demanding information, but the sarcasm and fear prevent any breakthroughs. Gideon decides they must 'force their hand' to reveal the perpetrator.

Gideon announces that two people are dead and sets a trap: he will act as bait in a sleeper cabin. The plan is for Vivien and Gavin to monitor the carriages while others stay locked in their cabins. While alone, Gideon hears movement, and a shadowy figure attacks him, leaving another note: 'You were warned.'

Gideon, Vivien, and Gavin regroup, discussing the escalating notes and the shadowy presence. Gideon deduces the notes are becoming more urgent and that the entity is always a step ahead. He realizes the answer lies in the engine room, specifically with the "light" mentioned in a previous note. Back in the engine room, they find a mannequin in the driver's seat with a new note: 'The answer lies in the light.'

Following the clue, Gideon activates a glowing panel in the engine room, transporting them to a hidden metallic corridor lined with pulsating lights. This corridor leads to a vast chamber with consoles and screens displaying strange symbols. At its center stands a pedestal with a device glowing blue, which Gideon touches. A mechanical voice, belonging to "The Architects," warns them that "the balance has been disrupted" and they "must restore it" or "choose."

They are returned to the metallic corridor, now covered in glowing symbols. They enter a vast chamber with a massive column of swirling light. Around its base are the missing passengers, their eyes glowing blue and movements unnatural. One, the woman in the tailored suit, speaks mechanically, explaining they are being "realigned" to restore balance. Gideon understands they face a choice: submit to this force or fight it, risking everything.

Gideon concludes that the entity wants control, not balance, and decides to fight. He instructs Vivien and Gavin to find the device that controls the system. As the glowing passengers become aggressive, Gideon engages them while Vivien and Gavin frantically work at the glowing panel. Gavin successfully activates a command, and a blinding light engulfs the room, followed by silence. They find themselves back in the lounge car, the other passengers seemingly normal, and the storm subdued. A final note reads: 'The cycle is broken… for now.' Gideon feels the real battle is yet to come.