After Gisella reveals that Varlos is her father the Doctor promises her a family reunion. She tells him that Varlos went to Paris in 1895 and settled in Montmartre. The Doctor pilots the TARDIS to this destination. On the way through the time vortex he realises that something is following them but gets shaken off just as they arrive.
In a squalid orphanage, nine year-old Nicholas is woken by a scraping noise. He looks around his dormitory and sees a monster snapping at the children with huge jaws and lasing out with tentacles. As two crimson eyes focus on him he screams.
The Doctor and Gisella step out into the starlit streets of Paris. The pavements are unusually deserted except for an old man of about seventy looking at his reflection in a shop window. He turns to them and announces that he is Nicholas and he is nine years old. He adds that his orphanage has just been attacked by a monster and all of the children have suddenly aged like him. The Doctor takes him to the nearest police station. They find a crowd of old people huddled inside, telling a similar story to Nicholas. Two policemen are discussing the events at the circus where the animals were aged to bones between performances. The Doctor shows them his psychic paper that informs them he is Doctor LeSmith of the Ministry of the Interior. Gisella asks the Doctor how she can speak the local language and feel the words being translated in her mind. The Doctor tells her about the TARDIS’s translation device but is impressed that she can detect the process.
Passing through the crowd the Doctor aims himself towards Inspector DuPont’s office. He is intercepted by the Inspector’s advisor, Baron De Guerre. The Baron is dismissive of the Doctor’s usefulness to this case and critical that he should have such a young girl as Gisella with him. The Doctor blusters his way into the office where the Inspector shows him a paper detailing the events of the night. While a lot of the children have been aged to their seventies some have apparently become centenarians and one middle-aged woman is now a toddler again. The Doctor speculates that this is not a chromovore in action, the results are too erratic, but he likes the Inspector’s description of the creature as a time vampire.
Heading back to the TARDIS to get some equipment to track time decay in the area, the Doctor is confronted by the Darksmiths’ robot Agent. From the heavy damage that the robot has sustained it is obvious that this was the object that tracked him through the vortex. The Agent says that it managed to find a puncture in the fabric of space time and made a crash landing after it lost the TARDIS but it claims it can still kill the Doctor. However, when it tries to shoot him its energy beam misfires. The Doctor uses the time to ask why the Agent wants to kill him when only he knows where the Crystal is hidden but the Agent says that it has detected Varlos’s bio-signs in the city and it will use Gisella as a bargaining counter to get what it wants.
The Doctor runs away from the Agent and finds De Guerre and Gisella waiting for him by the bones of a recently deceased woman. He tells Gisella about the Agent’s plan to catch her but says he needs to investigate the hole in space-time; it may be connected to the time vampire. They head off towards De Guerre’s apartment to phone DuPont about the latest death.
The apartment is grand but rather untidy. They decide to retire to bed for the remainder of the night and the Baron and Gisella leave the Doctor in the sitting room. He takes the opportunity to snoop around and is intrigued by an old wooden chest, secured by a padlock. He wonders why the Baron would leave valuables lying around in plain sight but lock something else away. Before he can do anything the telephone rings and the three investigators learn that the vampire has been cornered near the Eiffel Tower.
police carriage takes them to the tower. Gisella ponders the reason why the vampire should have left its normal stalking grounds around Montmartre and crossed the city. The Baron says it needs to be sent back to where it came from but when the Doctor asks where that is he gets no reply. The Doctor suspects that De Guerre knows more than he is letting on.
They arrive at the tower to find it surrounded by police. The Doctor uses DuPont’s binoculars to look at the monster on the second stage of the tower: it is a dark, hairy beast with matted hair. The beast has blocked the stairways so the Doctor climbs up the outside of the tower. When he reaches the creature he finds that it resembles an ape more than a man and seems incapable of answering his questions. It launches itself at the Doctor and tries to throw him off the tower but it is killed by a shot from a police marksman and drops him on the right side of the barrier.
Later, DuPont claims that the creature was an escaped zoo animal and that Paris is now free of the vampire. The Doctor tells Gisella that whatever it was, the ape was not the vampire that attacked the orphanage. To prove this they find Nicholas and take him down to the police cell where the creature’s body is lying. He confirms that this was not what attacked him. They notice scraps of blue cloth on the creature’s body; similar to the overalls worn by the Eiffel Tower’s cleaners, and in one of the pockets is an employment card for Pierre Breton. The Doctor guesses that Breton was attacked by the vampire but instead of growing older he was transformed into something millions of years earlier in the evolutionary timeline. It makes him think that the vampire is cloaked in a protective time shield that is somehow misfiring and creating erratic effects when people are attacked.
They decide to take Nicholas to the Baron’s apartment to convince him of the true nature of the dead beast and try to get him to persuade DuPont that the vampire is not dead. When they arrive the Baron is not at home but there are signs that he only recently left. The Doctor notices that the padlocked chest is now open. Inside are tools that Gisella recognises as having once belonged to Varlos and a bleached skeleton that they assume must be the Darksmith’s remains. Their conclusion is that the Baron and the time vampire are one and the same. The Doctor looks at the equipment and decides to use it to make a vampire-tracker but Gisella suddenly sees the Agent arrive outside the building.
When the Agent shuffles past without stopping the Doctor realises it must be tracking de Guerre’s Darksmith technology as it hunts for Varlos. They follow the Agent towards the Basilica of the Sacre Coeur which is being built on the hill overlooking Montmartre. The Agent pauses to look in a construction workers’ shelter before lumbering on. The Doctor stops to look in too and Nicholas feels a draft coming from behind the panels of the shelter. He pulls a panel away and finds a tunnel going into the hillside, too small for the Agent to have used but large enough for humans. Clambering down a passage they arrive in an enormous cavern. Inside is a huge stone spaceship of the sort used by the Darksmiths. Gisella recognises it as the one she and Varlos used to escape in. beside it stands another spaceship of a different construction. Neither the Doctor or Gisella recognise the design but Nicholas recognises the scraping sound of the vampire crossing the cavern. The Doctor tries to speak to the vampire but it keeps coming towards him so the three of them run into the second spaceship and seal the door. From the tracks on the floor it is apparent that this is the vampire’s craft.
In the cargo hold are weapons from many times and places in the universe. Sitting among them is the Baron. He tells them that the ship’s crew was made up of temporal arms dealers but the vampire is only their means of fuelling the ship. It collects time energy from wherever the ship lands and then loads it into the engines. Gisella accuse De Guerre of killing her father but his appearance shimmers and he reveals his true form: he is Varlos. He explains that the real De Guerre was the first victim of the vampire and the newly arrived Varlos decided to pose as the dead man to sort out the business quickly before the presence of futuristic technology on Earth alerted the Darksmiths. Nicholas comments on the unlikely coincidence of two spaceships arriving in the same place at the same time.
Varlos sadly explains that it is not a coincidence. He says that as he fled towards Earth (only weeks earlier for him) he saw a small hole in the space-time vortex and steered through it to the past. Unfortunately his ship must have widened the hole so that when he returned to his craft that morning to collect some equipment he was surprised to find another ship parked next to it. The crew of the ship died in the landing but the vampire set out on its mission to collect more fuel. Because the engines were already full the vampire had no way to offload its surplus so it carried on gathering fuel but leaking out even more and creating bizarre temporal effects. The Doctor predicts that a temporal explosion could result, blotting out billions of lives.
The only solution is to fit a chronon filter but to do so would require a sophisticated computer. The drawback is that there is nowhere for the excess time energy to go. Varlos then pulls out a replica of the Eternity Crystal, not as powerful and booby trapped, that he has been carrying in case the Darksmiths capture him. He says this can store the energy. An explosion on the hull announces the arrival of the Agent demanding the surrender of what it thinks is the real Crystal.
The four of them use an escape hatch to leave the ship and run up a nearby tunnel. The Agent follows them and before they can get to the outside air they realise the vampire is waiting for them near the exit. The Agent fires several shots up the tunnel and one hits the vampire. Suddenly, the tunnel, weakened by leaked temporal energy, collapses under the Agent and it plunges down into a pit. The vampire starts to glow as its shield, weakened by the robot’s shot, begins to fail.
The Doctor says he can use his own body to act as a conduit to transfer the time energy to the Crystal but Varlos says it needs a Darksmith’s brain to connect to the Crystal matrix. As the vampire slithers into the streets outside Gisella gives chase. Dozens of revelers, celebrating what they thought was the demise of the vampire, are horrified to see it coming towards them. It staggers into a tree which ages and dies in a second. The Doctor demands the Crystal but Varlos and Gisella run past him towards the vampire. They plunge into the haze around the vampire and bolts of lightning fly out of Gisella. One hits Nicholas and he reverts to being nine again. The tree returns to life, too. The Doctor realises that Gisella’s bolts are flying across Paris, returning everyone the vampire touched to the correct age. At last the vampire fades away, free to roam the vortex. The Doctor races to Gisella. Her skin has burned away to reveal her circuitry. Varlos tells the Doctor he was not the girl’s father but her creator. He lets the Doctor take the replica Crystal, saying that he is glad his last act was to save lives rather than saving the dead. Before he dies he asks the Doctor to destroy the Crystal for him.
Gisella repairs herself quickly before DuPont arrives asking if the real vampire is finally dead. She tells the Doctor that even the animals as well as the humans that the vampire killed have been restored to life.
Later the TARDIS materializes in the cavern and the Doctor sends the vampire’s ship and sends it into the vortex to roam forever. Gisella and the Doctor stand in the open door of the TARDIS as the Agent clanks up. It says that it has sensed the death of Varlos. It demands the Crystal but the Doctor says he has learned from Varlos where he needs to go to destroy the Crystal they shut the doors and head out to the Asteroid Belt. Gisella reminds the Doctor that Varlos told them nothing about destroying the Crystal but the Doctor watches the Darksmith ship fly past. He says that the Agent is probably going to the place they need to find and they are going to follow him.
Sadly, as they try to tail the Agent they hit a slipstream of destronic particles: Varlos’s last trap for anyone who tried to follow him. The TARDIS begins to shake and the Doctor says it will crack open like an egg.