In the year 2047, faint radio signals from the starship Event Horizon are picked up on Earth. The ship had disappeared mysteriously in 2040; her loss was attributed to an accident. The ship appears to be in a decaying orbit around the planet Neptune, and the rescue ship Lewis and Clark is dispatched to investigate. The crew, which includes the Event Horizon’s designer, Dr. William Weir (Sam Neill), must spend the 56-day journey to Neptune in stasis in order to reach the Horizon.



When the Lewis and Clark reaches Neptune's high atmosphere, the crew come out of stasis. It is here that they are informed by Dr. Weir of the true story behind the "accident" which befell the Horizon. The ship had been a top secret prototype vessel equipped with an experimental stardrive. The drive, which would fold space-time and create an artificial wormhole extending to any point in the universe, allowed the ship to traverse great distances instantaneously. To test the new gravity drive, the ship was sent to a designated jump-point with the goal of reaching Proxima Centauri (the next closest star to our Sun); it subsequently vanished without a trace.

Upon approaching the drifting vessel, the crew scan for life signs aboard the Event Horizon. No definitive trace of human life is found; instead, it seems to be mysteriously abandoned, although strange, incomprehensible readings come up on the Clark’s sensors, which leads the crew to assume an equipment malfunction and perform a manual search for Event Horizon’s crew.

The crew board the ship to find that its artificial gravity and heating systems are inoperational. Engineer Justin (Jack Noseworthy) eventually reaches the Event Horizon’s gravity drive. Suddenly, the drive activates itself, as though alerted to his presence. Justin sees a black, liquid-like mirror within the drive; he curiously touches it. Without warning, he is sucked into the drive and a large shock wave flows through the ship, damaging the Lewis and Clark. Cooper (Richard T. Jones) manages to pull Justin out of the liquid, but he is catatonic. With the Lewis and Clark heavily damaged, Captain Miller (Laurence Fishburne) has no choice but to transfer the entire crew over into the Event Horizon itself. Although they successfully reactivate the Horizon’s life support, heating, and artificial gravity systems, the rescue crew is now in danger themselves, as there are only 20 hours worth of usable oxygen onboard the Event Horizon.

Once onboard the Event Horizon, the rescuers almost immediately begin to experience macabre visions, which bring to life their inner demons and personal guilt. Captain Miller (Fishburne) sees the manifestation of a subordinate whom he was forced to leave to die in a fire onboard a ship on which he previously served; another crewmember sees images of her son, with his legs covered in sores. Dr. Weir, the ship's designer, is assaulted by images of his deceased wife Claire, who committed suicide (there is some indication that Weir's constant absence from their home due to his work contributed to her decision to kill herself). She urges him to "join us" so he can be reunited with her.

The crew continue to explore the ship, and discover the truth behind its disappearance; the ship had indeed succeeded in opening a gateway in space-time by using the gravity drive. However, when it made the jump, it passed outside the known universe and into another dimension; "a dimension of pure chaos, pure evil." This dimension is portrayed as being analogous to the Judeo-Christian Hell. Upon inspection, the ship's highly degraded video log shows the ship's original crew engaging the gravity drive and moments later, committing an orgy of torture, cannibalism, rape, and general depravity. At the end of the recoverable portion of the log, the captain of the Event Horizon’s crew, who has torn out his own eyes, leaves an initially misinterpreted warning in Latin which, at first glance, appears to be Liberate me ("Save me"). A clearer interpretation later reveals him to be stating Liberate tutemet ex inferis ("Save yourself from Hell").

It eventually becomes clear that wherever it was that the ship has been, it has brought back with it a presence which is methodically trying to drive its new occupants insane by using their own personal torments against them. The aim of this appears to be to compel them to re-engage the gravity drive and "go back" to the chaos dimension. Captain Miller decides to destroy the ship to prevent this from happening; when he informs Dr. Weir of his plan, the evil presence onboard delves into Weir's mind, turning him against the rest of the crew in order to help it complete its mission. The presence cruelly forces Weir to re-live Claire's horrific suicide, which finally drives him over the edge, turning him completely and utterly demented. He, like the deceased captain before him, gouges his eyes out and proudly announces "where we're going, we won't need eyes to see". Enlisting his aid, the ship uses Weir to destroy the Lewis and Clark and then brutally kill several of the crew members who try to stop him from activating the gravity drive.

Dr. Weir eventually dies by being blown out into space by a ruptured window, as the possessed shipbuilder tries to convince Miller and his surviving comrades that it is pointless to resist. However, before this happens, he manages to engage the ship's gravity drive on a 10-minute countdown to initiation. Miller makes an attempt to set off the explosives which will sever the long 'neck' which connects the ship's control decks at the bow from the stern-mounted gravity drive and sublight engines; he manages to arm the explosives, but is forced to rush to the gateway room after almost being roasted by his personal demon: the former comrade who burned to death under his command. The doors of the room close just in time, but as he turns, he sees the burned man standing in front of him once again. This time, Miller manages to overcome the vision and the flaming figure dissipates, revealing Dr. Weir, who has been brought back by the ship:

"When she crossed over, she was just a ship, but when she came back, she was alive"

"The ship brought me back. I told you she won't let me leave. She won't let anyone leave".

Completely possessed by the evil presence on board, Dr. Weir gleefully informs Miller that it is now "time to go back". He explains to Miller that he should expect to have a thoroughly unpleasant time because "Hell is only a word. The reality is much, much worse." He then goes on to show Miller what awaits him in Hell. Images of the former crew being brutally tortured and all manners of violence being inflicted on their helpless, writhing forms fill Miller's mind. "Do you see?!!" asks Dr. Weir, thinking that he has won. "Yes… I see," says Miller, opening his hands to reveal the detonator for the explosives. Weir cries out too late as the Horizon explodes at the neck and separates in two, with their half plunging toward Neptune. Moments later, the gravity drive is activated, and Dr. Weir and Captain Miller are dragged back into the chaos dimension, plunging into a gateway in Neptune's atmosphere. Starck and Cooper, together with the comatose Justin, had remained in the forward half of the ship, and survive the ordeal - Starck whispers sorrowfully, "Miller." When a second rescue team arrives for them, one of the crew (Richardson) sees the face of Dr. Weir as a soldier lifts up his mask to greet him. As Starck snaps out of this delirium, it is revealed that they have been rescued. As the scene pans out, the doors of the ship close, leaving the viewer wondering whether the evil onboard the Event Horizon was ever truly destroyed.