The year is 1968, and as the BBC rebroadcasts episodes of the classic SF serial “Nightshade”, the townsfolk of Crook Marsham prepare for a lonely Christmas. At the local retirement home, actor Edmund Trevithick learns that a reporter is coming to interview him about his role as Professor Nightshade, and goes to sleep dreaming of past successes. But later that night, the scientists at the local radiotelescope are baffled by a sudden energy surge from an unknown source, which floods their instruments and blots out the signals they were monitoring from a nova in the vicinity of Bellatrix. Trevithick wakes to find that his window has been smashed open, and he faints when an evil voice in the darkness hisses the name of Professor Nightshade. Meanwhile, Jack Prudhoe is drowning his sorrows at Lawrence Yeadon’s pub, thinking back on the failure he’s made of his life, when his wife Win runs by the window -- young again, and as full of life as she was before the death of their young son crushed her spirit. Almost delirious with joy, Jack follows her out of the village to the moor… where something horrible happens.

The TARDIS materialises in Crook Marsham as dawn breaks. The Doctor is in a pensive mood, and he shouts angrily at Ace when she finds his grand-daughter Susan’s clothing and dresses in it as a joke. Telling Ace that he needs to think about things for a while, he sends her off to explore the town while he visits the local monastery to reconsider his self-appointed role as guardian of the cosmos. Ace meets Robin Yeadon, the pub owner’s teenage son, and becomes curious when Vijay Degun, a technician from the radiotelescope, comes in search of a working telephone only to find that the entire town’s phone system is down. Seeking excitement, Ace hides in the back seat of Vijay’s car and is taken to the radiotelescope, and while exploring she finds a guard’s rapidly decomposing body near a hole in the fence. She enters the building to tell the others, but nobody believes her claim; particularly not the head of research, the racist Professor Hawthorne.

Lawrence Yeadon’s wife Betty goes into hysterics while preparing for a bath, claiming to have seen her dead brother Alf climbing out of the water. She has always blamed herself for his death during the war, since she feels she shamed him into enlisting. Lawrence sends Robin to fetch Doctor Shearsmith, but Robin finds Shearsmith’s offices empty and instead goes to the old folks’ home to ask Jill Mason for help. Jill is seeing off her charges as they leave town to visit their families, and Constable Lowcock is questioning Trevithick about the previous night’s incident. He and Trevithick accompany Robin back to the pub, where Lawrence tells Robin to take care of Betty while he and Lowcock fetch help from the next village over.

The Doctor spends some time with Abbot Winstanley, and reads up on the history of the village. During the Civil War, Marsham Castle was completely destroyed as if by heavenly fire, on the very ground on which the radiotelescope was later built. Not terribly concerned, the Doctor returns to the pub to fetch Ace, only to find the village in a turmoil as more villagers are found to have vanished during the night. Trevithick tries to interest the Doctor in his story, but the Doctor doesn’t want to get involved and decides to look for Ace at the radiotelescope. Robin overhears him and decides to accompany him, since Betty seems to be sleeping normally now and Robin wants to see more of Ace. However, soon after they leave, there is another surge of energy at the radiotelescope -- and Betty awakens to find her dead brother standing outside her door. Trevithick, on his way back to the retirement home, is attacked by one of the insectoid aliens from the first “Nightshade” serial, but it vanishes into thin air when Lowcock and Yeadon return -- retching and fainting, and claiming that they were unable to leave the village.

When Vijay goes to fetch his lover Holly to help analyse the readings from the array, he finds the ghost of James, her dead lover, sitting at the edge of her bed. James’ image dissolves into a fountain of light which nearly consumes Holly before Vijay manages to snap her fully awake. Meanwhile, the old folks’ bus crashes on its way out of town when the driver falls victim to the same force that prevented Lowcock and Yeadon from leaving. Tim Medway, the BBC reporter coming to interview Trevithick, feels no such influence when he enters the town, and when he comes across the accident he helps Jill to evacuate the stunned old folk to the monastery. Medway stops at the police station to report the accident, only to find them fully occupied by the disappearance of so many townspeople. He continues on to the pub, where Trevithick fills him in on the unbelievable events and decides to alert the Doctor, who seemed to believe his story. Medway, unable to accept what he’s stumbled into, nevertheless agrees to drive Trevithick to the radiotelescope to look for the Doctor.

The Doctor and Robin find Jack Prudhoe’s decomposing body on the moors, and continue on to the radiotelescope with some urgency. Once he’s sure Ace is safe, the Doctor studies the readings the scientists have been taking, but can make no sense of them. He and Ace decide to redirect their investigation towards the history of the town, and go to the monastery while Robin returns home. There, he finds that Betty is dead, her body decomposing like the others they have found. When his furious father accuses Robin of abandoning her, Robin bolts from the pub to join Ace back at the monastery. There, the Doctor and Ace find that not only was the radiotelescope built on the grounds of Marsham Castle, but an archaeological dig in the same area was abandoned at the turn of the century for unknown reasons. The Doctor returns to the radiotelescope, leaving Ace at the monastery, but Hawthorne scoffs at the Doctor’s claims, accuses Holly and Vijay of taking psychedelic drugs, and storms off to his room -- where he is attacked and consumed by a Tar Baby, the embodiment of his childhood fears.

The old folk at the monastery try to raise their spirits with a singalong, but raise entirely the wrong spirits; the songs remind them of lost friends and family, which come to life around them. The ghosts transform into blazing fountains of light which consume the terrified seniors and monks, including Winstanley. Jill flees in terror, while Robin and Ace, cut off from the exit, have no choice but to climb the stairs and try to hide in the attic. There, they are trapped by Billy Coote, a homeless old man who sleeps here on cold nights -- and who has been possessed by the Sentience which is eating the villagers’ lives.

The Doctor sees the blazing light from the monastery and rushes off to rescue Ace. He runs into Jill on the way, and orders her to gather the surviving villagers together in the church. Medway and Trevithick arrive at the radiotelescope soon after, but Medway panics and flees upon finding Hawthorne’s decomposing body. When he tries to drive out of town, however, he too falls victim to the fear barrier, which let him in but will not let him out. When he tries to push through regardless, he nearly runs over the fleeing Jill Mason, and is killed when he swerves off the road to avoid her. Back at the radiotelescope, Holly, Vijay, Trevithick and Dr Cooper are attacked by the insect monsters from “Nightshade”, and are forced to split up. The monsters pursue Trevithick into the depths of the complex, where he manages to destroy one in a lift shaft, using his old service revolver to blow up a fire extinguisher. He then sees a fierce, glowing light creeping up the lift shaft towards him, but manages to get out of the shaft and escape before it consumes him.

The Doctor finds the monastery littered with decomposing bodies -- and then his grand-daughter Susan steps out of the shadows towards him. He flees from her to the attic, where he finds Ace and Robin trapped with Billy Coote. The Sentience which possesses him is barely sentient at all, and knows only an all-encompassing hunger. Billy’s body breaks into shafts of light which attempt to consume the Doctor, Ace and Robin, but Ace distracts it using her nitro-9a capsules. The Doctor, Ace and Robin flee as the attic explodes, but the Sentience feeds upon the energy released by the explosion and grows stronger yet. Meanwhile, Jill and Lowcock get the survivors to the church, although Lawrence Yeadon and Win Prudhoe have both fallen victim to the Sentience in the form of their dead spouses. As the frightened villagers gather in the church, the strengthened Sentience rushes over the moor to the village -- and an old veteran studying the war memorial on the church wall inadvertently provides it with another form. The villagers find their church under siege by dead soldiers, who begin to batter through the doors and windows to get at them.

The Doctor, Robin and Ace return to the radiotelescope, where they find that subsidence near the breach in the fence has exposed the old Paleolithic quarry and archaeological site. The Doctor finally realises that the energy surges which have been flooding the instruments are not from space at all, but from beneath the ground. He takes Holly, Vijay and Trevithick to confront the Sentience on its own turf, but it has grown too strong for them, and consumes both Holly and Trevithick, forcing the Doctor and Vijay to retreat. The Doctor seems powerless to defeat it, but Ace realises that the Sentience has been taking forms from its victims’ memories because it requires them to submit before it can consume them. She proves her theory by summoning the Sentience to her with her own memories of her mother, and then banishing it by refusing to concede to her mother’s hold over her emotions.

The Doctor leaps into action and has Cooper realign the radiotelescope until it is once again picking up the nova in Bellatrix which the scientists were originally studying. He then summons the Sentience in the form of Susan, and shows it the nova -- a source of more energy than it could ever have imagined. The Sentience tears itself free of the earth, leaving Crook Marsham at last. As the Doctor prepares to follow, Ace asks his permission to stay behind with Robin, but the Doctor asks her to take one more trip with him first. They follow the Sentience as it travels back in Time, in order to reach the star before it goes nova, and watch as its departure from Earth tears apart Marsham Castle, to the horror of the watching Roundheads and Cavaliers. The Sentience reaches the star in Bellatrix just as it goes nova, and gorges itself on the release of energy. Still not sated, it locates another supernova in a distant galaxy, travels there and settles down to feed… and the Doctor and Ace watch with satisfaction as the star collapses into a black hole, trapping the Sentience forever. Ace now expects to be taken back to Crook Marsham, but the Doctor has another agenda for her, and instead he vanishes into the TARDIS corridors, refusing to acknowledge her pleas to be taken back.