The crew of the space shuttle Churchill finds a 150-mile long spaceship hidden in the corona of Halley's Comet. The crew finds hundreds of dead, shrivelled bat-like creatures and three naked humanoid bodies (two male and one female) in suspended animation within glass containers. The crew recovers the three aliens and begins the return trip to Earth.
During the return journey, mission control loses contact with the shuttle and a rescue mission is launched to investigate. The rescuers discover that the Churchill has been severely damaged by fire, with its internal components destroyed, and the three containers bearing the aliens are all that remain intact. The aliens are taken to the European Space Research Centre in London where they are watched over by Dr. Leonard Bukovski (Gothard) and Dr. Hans Fallada (Finlay). Prior to an autopsy, the female alien (May) awakens and drains the "life force" out of a guard. The female alien then escapes the research facility and proceeds to drain various other humans of their life force, revealing an ability to shape-shift. It transpires that the aliens are from a race of space vampires that consume the life force of living beings, rather than their blood.
Meanwhile, in Texas, an escape pod from the Churchill is found, with Colonel Tom Carlsen (Railsback) inside. Carlsen is flown to London where he describes the course of events, culminating in the draining of the crew's life force. Carlsen explains that he set fire to the shuttle with the intention of saving Earth from the same fate and escaped in the pod. However, when he is hypnotized, it becomes clear that Carlsen possesses a psychic link to the female alien. Carlsen and SAS Col. Colin Caine (Firth) trace the alien to a psychiatric hospital in Yorkshire. While in Yorkshire, the two believe they have managed to trap the alien within the heavily sedated body of the hospital's manager, Dr Armstrong (Stewart); but Carlsen and Caine later learn that they were deceived, as the aliens had wanted to draw the pair out of London.
As Carlsen and Caine are transporting Dr Armstrong in a helicopter back to London, the alien girl breaks free from her sedated host and disappears. When they reach London a "plague" has overtaken the city and martial law has been declared. The two male vampires, previously thought destroyed, have escaped from confinement by shape-shifting into the soldiers guarding them; the pair then transform most of London's population into zombies. After their life force has been drained by the male vampires, the victims seek out other humans in order to absorb their life force, perpetuating the cycle. The absorbed life forces are channeled by the male vampires to the female vampire, who transmits the accumulated energy to their spaceship in Earth's orbit.
Fallada impales one of the male vampires with an ancient device of "leaded iron". Carlsen admits to Caine that, while on the shuttle, he felt compelled to open the female vampire's container and to share his life force with her. She is later found inside St. Paul's Cathedral, lying upon the altar, transferring the energy to her spaceship. She reveals, much to Carlsen's shock, that they are a part of each other due to the sharing of their life forces, thus their psychic bond.
Caine follows Carlsen into the cathedral and is intercepted by the second male vampire, whom he dispatches. Carlsen impales himself and the female alien simultaneously. The female vampire is only wounded and returns to her ship with Carlsen in tow, releasing a burst of energy that blows open the cupola dome of St. Paul's. The two ascend the column of energy to the spaceship, which then returns to its hiding place within the comet.
What is the female alien's name?
May