In 2047, a distress signal is received from the Event Horizon, a starship that disappeared during its maiden voyage to Proxima Centauri seven years previously that has mysteriously reappeared in a decaying orbit around Neptune. The rescue vessel Lewis and Clark is dispatched. Its crew – Captain Miller, second-in-command Lieutenant Starck, pilot Smith, medical technician Peters, engineer Ensign Justin, Doctor D.J. and rescue technician Cooper – is joined by Dr. William Weir, who designed the Event Horizon. He briefs the crew on the ship’s experimental gravity drive, which generates an artificial black hole and uses it to bridge two points in spacetime, reducing travel time over astronomical distances. The distress signal seems to consist of a series of screams and howls but D.J. believes he can discern the Latin phrase “Liberate me” (“Save me”) being spoken.
Upon boarding the Event Horizon, the crew finds evidence of a massacre. As they search for survivors, the ship’s gravity drive activates, briefly pulling Justin into the resulting portal and causing a shock wave that damages the Lewis and Clark, forcing the entire crew to board the Event Horizon. Justin emerges in a catatonic state, terrified by what he saw on the other side. He attempts suicide by decompression, but is saved by Miller, forcing the crew to place him in stasis.
The team begins to experience apparitions of individuals from their past that only they can see, hallucinations corresponding to their fears and regrets: Miller sees Corrick, a subordinate he was forced to abandon to his death, Peters sees her son with his legs covered in bloody lesions and Weir sees an eyeless vision of his late wife, who committed suicide, urging him to join her. The crew soon discover a video log of the Event Horizon’s crew fornicating and mutilating each other shortly after first engaging the gravity drive. The video log ends with a shot of the Event Horizon’s captain speaking the complete Latin phrase from the earlier distress call, which D.J. translates “Liberate tutemet ex inferis” (“Save yourself from hell”).
Deducing that the ship’s gravity drive opened a gateway to a hellish dimension outside the known universe, and that the Event Horizon has somehow attained sentience, Miller decides to destroy the Event Horizon and orders an evacuation. Peters is lured to her death by a hallucination of her son. Weir, who has gouged his own eyes out and is now possessed by the evil presence, uses an explosive device to destroy the Lewis and Clark. The explosion kills Smith and blasts Cooper off into space. Weir kills D.J. by vivisecting him and corners Starck on the bridge. Miller confronts Weir, who overpowers him and initiates a 10-minute countdown until the Event Horizon will return to the other dimension.
Cooper, having used his space suit’s oxygen supply to propel himself back to the ship, appears at the bridge window. Weir shoots at him and is blown into space by the ensuing decompression. Miller, Starck, and Cooper survive and manage to seal off the ship’s bridge. With their own ship destroyed, Miller plans to split the Event Horizon in two and use the forward section of the ship as a lifeboat. He is attacked by manifestations of Corrick and a resurrected Weir. Miller fights them off and detonates the explosives, sacrificing himself.
The gravity drive activates, pulling the ship’s stern section into a black hole. Starck and Cooper enter stasis beside a comatose Justin, and wait to be rescued. Seventy-two days later, the wreckage of the Event Horizon is boarded by a rescue party, who discover the remaining crew still in stasis. Starck sees Weir posing as one of the rescuers and screams in terror; this is revealed to be a nightmare with Starck waking up moments later. Cooper and the rescue team comfort the newly-awakened and terrified Starck as the bulkheads close.