On August 9, 1985, during the extraordinary occurrence of Halley's Comet, a joint British-American mission aboard the space shuttle Churchill finds itself on an unprecedented journey. As they get closer to the comet, the crew makes a shocking discovery: a massive 150-mile long alien spaceship concealed within the comet's tail. Their curiosity piqued, they venture inside the spacecraft, uncovering not only hundreds of deceased bat-like creatures but also three naked humanoid bodies—two males and one female—suspended in glass coffin-like containers. With no time to waste, the crew successfully recovers these aliens and begins their return journey to Earth.
However, as they return, calamity strikes. Mission Control suddenly loses contact with the shuttle, prompting an urgent rescue mission to ascertain the fate of the crew. When the rescuers arrive, they are greeted by a harrowing scene: the Churchill is left charred and gutted by fire, with only the three alien specimens remaining intact. These aliens are subsequently transported to the European Space Research Centre in London, where they are closely monitored by scientists Dr. Leonard Bukovsky (Michael Gothard) and Dr. Hans Fallada (Frank Finlay). Just before a planned autopsy, the female alien, a seductively dangerous entity known as a 'vampire' (Mathilda May), awakens and swiftly drains the life force of a guard, enabling her escape. She embarks on a path of chaos, draining humans of their energy and using various host bodies to evade capture. The two humanoid males unexpectedly awaken as well, wreaking havoc, attacking guards with ferocity, quickly leading to a perceived annihilation—yet their fate is far from over.
Meanwhile, in a twist of fate occurring in Texas, an escape pod from the ill-fated shuttle Churchill is discovered, revealing the only survivor, Colonel Tom Carlsen (Steve Railsback). Upon being flown to London, he recounts the terrifying ordeal of how the crew was methodically drained of their life forces before he made the desperate decision to torch the shuttle and flee in the pod. During a hypnosis session, Carlsen realizes he possesses a psychic bond with the female alien—a connection that will prove pivotal. Joined by Col. Colin Caine (Peter Firth), a skilled member of the SAS, they track the vampire to a hospital for mentally disturbed patients in Yorkshire, where they believe they have corralled the alien girl within the body of Dr. Armstrong (Patrick Stewart), the hospital's highly sedated manager. But as the plot thickens, it becomes evident that this is nothing more than a cunning ruse designed to mislead them.
While transporting the sedated Dr. Armstrong back to London by helicopter, the female alien cunningly breaks free and vanishes into the night. Carlsen confesses to Caine that he was irresistibly drawn to open the alien's container on the shuttle, sharing intimate moments that entwined their very life forces. As they reach the city, all hell breaks loose; a plague fueled by the vampires’ rampant energy consumption engulfs London, sparking waves of violence and panic. Even the Prime Minister has succumbed to this insidious infection, and martial law is executed to prevent the spread of this nightmarish contagion. The chaos escalates as the two male vampires break free from confinement and taint London’s populace, turning them into ravenous, zombie-like creatures with a single-minded hunger for life force energy. This plague transforms its victims into living-dead terrors every two hours, who in turn seek out fresh victims, perpetuating a cycle of horror.
With the military's assistance, Carlsen and Caine end up in quarantine at the city limits, desperate to implore the authorities to allow them to return to tackle the situation head-on. Realizing that their time is running out, they split up: Carlsen heads toward the iconic St. Paul's Cathedral, while Caine maneuvers back to the devastated research center, battling through hordes of both fleeing humans and insane zombie-vampires. Caine discovers Fallada in his office, who has bravely managed to kill one of the male vampires, using a lead sword—an ingenious insight that the vampire legend might trace its roots back to this alien race.
As events spiral into chaos, Carlsen becomes acutely aware of his psychic connection being used as bait, leading him back to the vampiric alien. He finds her lying on St. Paul’s Cathedral’s altar, retrieving energy to send back to her spacecraft. Caine follows the trail, confronting and defeating the second male vampire, but not without cost; he is forced to eliminate Fallada due to an infection that has ultimately turned fatal. In a desperate bid, Carlsen sacrifices himself, managing to impale both himself and the female alien. Though wounded, she manages to return to her ship, releasing an uncontrollable burst of blue energy into the sky, detonating the dome of the cathedral and taking Carlsen with her.
As the two ascend into the alien ship propelled by brilliant blue light, Caine is left amidst the ruins of London, a city ablaze yet finally free from the spell of the vampire-zombies. Reflecting on the remnants of tragedy, he witnesses the city fall still, though knowing the potential for another catastrophe looms—this mysterious racing spacecraft now dashes away towards its home in the cosmos, laden with the souls it has ensnared, a grim reminder of the horror that may one day return, precisely 75 years from this day.
