At the London Hospital, a patient is taken into surgery. After ensuring that Mr. Morgan, the patient, is comfortable, the doctor starts by examining the x-rays. Up above, two medical students are watching the proceedings. The patient will be conscious throughout the operation, as Dr. Magus needs to check his reactions. Dr. Magus asks for a scalpel as his assistant reports that the patient's pulse is normal and breathing is steady. Magnus applies electrolytic cream to the patient's forehead, and reports everything looks fine. The patient's pulse begins to increase. More cream is applied, before Cerebral Pulsator is activated. Magnus then applies the Pulsator's electrodes to the patient's head. This is the critical part; one mistake when applying the current could be disastrous. The Pulsator beeps away, signalling that all is good. The operation was a success.
On Cloudbase, the Spectrum personnel try to decipher the latest Mysteron threat. Unfortunately, all the information they have on time is over 14 microfiles long, an impossibly large amount of data to cover. They must first narrow the possibilities. As they don't know where the Mysterons will strike, field headquarters will be set up strategic cities. Captain Blue is assigned to New York, Captain Grey to Rio de Janeiro, Captain Ochre to Moscow and Captain Scarlet to London. Lieutenant Green has the flight deck cleared for the immediate departure of their Spectrum Passenger Jets. Green wonders if the Mysteron threat to kill time could be a trick, but White doesn't think so; he's sure they intend to destroy something or someone.
Outside the London Hospital, Dr. Magnus is preparing to leave, as is his assistant Turner. They are both pleased with the results of the operation as the patient is expected to make a full recovery. They will be doing a similar operation tomorrow. After his car is brought to the front entrance, Magnus drives off, closely followed by the sinister figure of Captain Black. Magnus pays a visit to the Westbourne Clinic where his next patient is staying. Captain Black watches from his car. Magnus arrives in the room of his patient, whom he addresses as "General". He confirms that the operation will be performed tomorrow, but he needs his signature on a release form, just as a formality as there will be little risk with the Cerebral Pulsator. The general signs the form, revealing his name to be J.F. Tiempo. Magnus leaves the clinic, and is again followed by Captain Black. Magnus becomes a little nervous when he sees Black's car following him so closely. He is forced to stop at a junction. Before the lights change, Black opens fire, narrowly missing the doctor's head and shattering the windscreen. Magnus doesn't wait for the light and speeds off. Black attempts to follow but is blocked by another car. Magnus begins to relax thinking he has escaped his pursuer, but not for long – Black is suddenly back on his trail and taking pot shots at him. Black manages to shoot out one of the tyres on Magnus's car just as it approaches a bend in the road. Unable to manoeuvre, Magnus crashes off the end of the cliff and is killed as his car explodes. On the rock above, a familiar face watches the wreck. It's Dr. Magnus! Or at least, his Mysteron duplicate. Captain Black reminds him of his mission – to kill time.
Destiny Angel returns to Cloudbase, not has nothing out of the ordinary to report. Later, Colonel White is having a quiet moment of stargazing and trying to work out the Mysteron threat, when the silence is suddenly shattered by Captain Magenta's overly enthusiastic radio call. Magenta believes he has discovered what the Mysterons plan to do, saying it is so obvious now. He had been reading the papers, when he spotted an article about a General Tiempo, the leader of the Western Region World Defence, who is in London for an operation. The general is an obvious target for the Mysterons he explains, as 'Tiempo' in Spanish means 'time'. 'Kill time' means kill Tiempo.
In his Westbourne Clinic bed, General Tiempo is fast asleep. The door silently opens and a figure walks in. It's Dr. Magnus, but luckily Captain Scarlet appears, stopping the doctor from doing anything. The exchange between the two men wakes General Tiempo, who is informed by Scarlet that his operation will be taking place on Cloudbase instead.
On Cloudbase, Dr. Magnus is furious that his operation has been moved without his consent. Colonel White tries to explain that Tiempo is a Mysteron target, but Magnus is still fuming. Dr. Fawn says that Cloudbase has the necessary facilities to perform the operation, but that they should postpone it for a few weeks. Magnus demands that they go ahead immediately. Colonel White allows the operation to go ahead, Tiempo is Magnus's patient after all.
Sometime later, Captain Blue reports that the other doctors have arrived on Cloudbase and the specialist apparatus required has been installed. Dr. Fawn will assist in the operation, while the others watch on the TV cameras. The general is currently getting his x-rays by a somewhat annoyed Dr. Magnus, who keeps getting his hands in the way of the x-ray machine. Everything is now ready, but Colonel White has a feeling that the Mysterons will soon strike.
In the operating room, the patient is ready for the operation, but the x-rays haven't arrived yet, further annoying Dr. Magnus. The radiographer himself gets quite annoyed when he discovers that Magnus's hands are obscuring one of the x-rays. With the x-rays having finally arrived, Magnus begins the operation. However, the radiographer notices something strange with the x-ray of Dr. Magnus's hand – it's not an x-ray, but a shadow of his whole hand and forearm. Back at the operation, Magnus has the Cerebral Pulsator switched on for the final part of the procedure, but has the voltage set far too high at 1.7. Magnus's x-ray now shows a full human hand as though it was taken with a proper camera. The radiographer realizes that something is very wrong. He tries to stop operation, but is prevented from doing so by Captain Magenta who is on guard duty outside the theatre. Not getting anywhere with Magenta, the radiographer rushes off the see Colonel White. The patient's pulse and breathing run out of control. Magnus is uncovered as a Mysteron, but triumphantly declares that they are too late and Tiempo is dead. However, Dr. Fawn reveals that he has actually killed Captain Scarlet. Outraged, Magnus throws a table at Dr. Fawn and runs out of the theatre, knocking Captain Magenta out as he goes. As Cloudbase is sealed off, Captain Blue hears shots and goes to investigate. Captain Grey reports that Magnus has locked himself in the generator room. Captain Blue goes in after him through the maintenance door. He creeps up on Magnus, who is still covering the other door with the gun he somehow acquired from Captain Scarlet. Magnus refuses Blue's offer of help to be free from Mysteron control. Out of options, Blue pushes a ladder towards Magnus, who, in jumping out of the way, backs onto a high voltage spike and is killed.
General Tiempo later thanks Colonel White and Spectrum for all they have done, but the colonel says it is what they're here for. White thanks Scarlet for taking Tiempo's place in the operation. Although Magnus was killed, they have learned two things about the Mysterons from this incident. One, they are impervious to x-rays and second, and high voltage electricity can kill them as well as men.