At St. Andrews hospital in Inverness, Scotland, young Mary Campbell lies near her death. She is an innocent victim of a madman’s quest for retribution. A nurse tends to the medical devices next to Mary’s bed, taking pity with her. Mary’s lifesigns are getting steadily weaker and the doctors fear she won’t survive the night. The nurse touches the forehead of Mary, who, in some sort of feverish dream, says “Bo.” Mary opens her eyes and the nurse is startled by the eerie glow that emanates from them. Mary yells, “Don’t hurt me,” and then several objects in the room start to float, all engulfed by the same strange glow that is coming from the girls eyes. Mary panics, screaming that the little people are coming for her and for someone to stop them. The nurse tries to calm down little Mary and eventually succeeds, causing all flying objects come crashing down. She wonders who or what Mary is that she can do such things.
In a secret lair, Moira MacTaggert stands before Master Mold. It is pleased with her performance, as the genetically engineered virus she created already claims its first victim with astounding rapidity. Soon, it will release the death spores all over the globe; Homo superior will wither and die, while humanity will flourish. The concept of irony is not beyond Master Mold, as it is well aware that the instrument with which he brings Retribution upon mutantkind is none other than Moira MacTaggert, geneticist spokeswoman for mutant rights and, in her time, paramour of two mutants.
The entranced Moira addressed the giant figure in front of her, informing it that, although Mary Campbell has contracted the virus, she would like to observe its pathology first hand. It would aid in Master Mold’s cause. For all they know, something could still go wrong at St. Andrews. Master Mold enjoys hearing such words from Moira’s mouth, as it shows how cold and emotionless he has turned her, but then it complains that she still refuses to call him Stephen. Master Mold states that it is Stephen Lang, a human mind trapped in this body, robbed of humanity by Scott Summers, the mutant called Cyclops.
Before Master Mold can continue its rant, it is interrupted by an incoming message. Referring to the voice on the other end of the line as Conscience, Master Mold orders him to speak. Conscience says that he received a report of Sean Cassidy having exhibited signs of the Retribution Virus and is currently being taken to hospital by Scott Summers. Master Mold is please, another mutant claimed rather quickly. It tells Moira that she will have not only one, but two subjects for her study.
In Scotland, Scott carries his unconscious friend into St. Andrews, calling for help. Dr. McKinnon and other medical staff place Sean Cassidy on a stretcher, while an agitated Scott tells them that he is burning up with a fever. The doctor says that he has just checked on the Campbell girl and, if Cassidy contracted the same virus, then he fears they might be facing an epidemic. Scott asks how Mary is but McKinnon has no good news, her condition is still deteriorating. However, he also tells Scott of the startling discovery that she is a mutant, for she manifested some telekinetic power in her room. The doctor comments that he never would have suspected, for she seemed such a normal girl. Scott replies that her being a mutant doesn’t make her any less an eight-year-old child, but then excuses himself, as he wants to stay with Sean, who is wheeled away by some nurses.
Elsewhere, Master Mold orders the Servitors to engage all systems, for they go to claim their victims. Across the night sky, a ball of light swings from Muir Island to silently land outside the St. Andrews hospital. A probe is launched, which enters the building and releases an ultra-high frequency beacon, rendering everyone inside unconscious within seconds. Outside, Master Mold emerges from the brilliant light, as do the Servitors, hundreds of artificial creatures, bald, naked, with bug-like eyes and half the size of an average human. Master Mold orders them to take Moira in hand and find the two mutant test subjects. The apparently mute creatures do as told and rush into the hospital.
Inside, Scott slowly comes around and finds himself face to face with hundreds of Servitors. He also notes that he feels the same evil presence he felt on Muir close by. Though he doesn’t know what the Servitors are, their intentions become pretty clear to him, once his eyes adjust to the bright light that everything is bathed in. The creatures have already retrieved Sean Cassidy and Mary Campbell and are already carrying to a nearby window. Scott tells them to put the two down, but he receives no answer. Only one Servitor turns and takes a good look at him, before continuing with their mission.
Outside, Master Mold receives a beacon call from the single Servitors and, allowing itself to see what it sees, it becomes aware of Scott Summers trying to interfere with his plans. However, it decides that Summers is too late; he cannot save his prey or even himself. Master Mold and all his followers prepare for their departure and walk towards the giant glowing sphere of light, which will take them back to their base.
Scott runs out of the hospital and sees the group marching towards the light. He doesn’t recognize Master Mold, though, for it has already enters the light. All Scott is able to make out is something immense in the center of the light and he has no time to worry about that, for he wants to catch up with the Servitors carrying Mary and Sean. However, he finds more and more of the creatures blocking his paths and he barely manages to keep them at bay with his optic beam. As more and more approach him, forcing Scott against a wall, he increases the intensity of his blast, knocking the Servitors over. The creatures scatter and then all race towards the light.
Looking after them, Scott becomes aware of Moira, who too is about to venture inside. Knowing that he can’t reach Sean or Mary anymore, Scott shouts that at least they won’t get Moira too and grabs her arm. The last of the Servitors being inside the glowing sphere, there is the roar of engines, a blinding dazzle of light and a savage hurricane of wind. Through it all, Scott holds the desperately struggling Moira MacTaggert. Once the effect is gone, she yells “My test subjects. What have you done!? I must be with them! I must -”
Scott shakes Moira, telling her to snap out of it. He tells her he is her friend and there for her. Moira finally comes to her senses and seeks comfort in Scott’s arms. Looking up to the departing light in the night sky, she says she has killed Scott and Mary and, may God forgive her, she killed them all.