The Brigadier, Jo and The Doctor

At the government-owned Auderly House a British diplomat, Sir Reginald Styles, is trying to organise a peace conference to avert World War III. However, while in his study a soldier dressed in grey camouflage and wielding a futuristic looking pistol bursts in and holds him at gunpoint. Luckily for Sir Reginald Styles before the guerrilla can fire, he vanishes, leaving Styles to shakily tell his secretary he has been visited by a ghost.

As the conference is of vital international importance, UNIT is called in to investigate. But when The Doctor, Jo Grant and The Brigadier arrive at Auderly House, Styles denies ever seeing the "ghost", even though The Doctor notes the presence of muddy footprints in the study. Outside a guerrilla reappears on the grounds in a vortex-like effect, but he is intercepted by two huge humanoid aliens, known as Ogrons, who attack him and leave him for dead. The severely injured guerrilla is soon discovered by UNIT who arrange for an ambulance to take him to a hospital while The Doctor examines his weapon and a small black box which were found in a nearby tunnel system.

As The Doctor examines both of these items he discovers the weapon is an ultrasonic disintegrator, made of Earth materials but not from this time period, and the small black box he recognises as a crude time machine, complete with a miniature dematerialisation circuit. As The Doctor examines the black box he activates it and in the ambulance the vortex effect appears again and the guerrilla suddenly vanishes.

Shura, Anat and Boaz

Since everything seems to be centred on Auderly House, The Doctor decides to spend the night there but the night passes without incident. However, in the morning three guerrillas appear from another time vortex - Anat, a woman who is in command of the mission, along with two men, Boaz and Shura – and make there way to the house killing a UNIT patrol on their way. In the study, The Doctor tries to reactivate the time machine again just as Shura enters the house. On spotting the time machine he pleads with The Doctor to turn off the box. Meanwhile in the 22nd Century a human reports that the machine has been activated. He is ordered to obtain the spacetime coordinates and that whoever is using that device must be exterminated…

Back in the present Anat and Boaz enter the study with Jo as their prisoner and demand that the machine be deactivated. With Jo being threatened The Doctor is forced to comply. It is then that The Doctor and Jo discover that the guerrillas have travelled back in time to assassinate Sir Reginald Styles because they believe he caused an explosion at the peace conference so starting a series of wars that left humanity vulnerable to attack - a history that they wish to change at any cost. Because the aliens who attacked Earth were the Daleks…

The Brigadier Instructs Captain Yates

On realising that The Doctor is not Styles, and that he has gone to Peking in an attempt to persuade the Chinese to rejoin the peace conference, Anat sends Shura to contact the future for more orders, but Shura only manages to retrieve a bomb from near the tunnel before he is attacked by some Ogrons. He is wounded, but he manages to stumble away. Back in the study Jo decides to stop the guerrillas – in doing so she accidentally reactivates the time machine and finds herself whisked away to the 22nd Century. Despite being told that Jo must have perished in the vortex, as the time machine is designed to be used by only its intended owner, he rushes after the two guerrillas who have returned to the tunnels. There The Doctor discovers a Dalek and in attempting to get away from it he locates Anat and Boaz just as they activate their time machines, and is swept up in the same vortex and so transported to the 22nd Century.

As The Doctor investigates his new surroundings he discovers the sorry state the human population is in under the Dalek’s rule and that most are being used as slave labour in factories. The Doctor is soon captured and is interrogated as it is thought he is a member of a guerrilla group. The Doctor is eventually brought before The Controller where he is reunited with Jo. Despite Jo believing that The Controller is a good person The Doctor knows that he is just a lackey for The Daleks and that he carries out all their orders regardless of the effects it has on his fellow humans.

What The Doctor does not realise is that on learning of The Doctors presence the Daleks react violently, declaring that The Doctor is an enemy of the Daleks and must be exterminated and that on learning this the guerrilla group decide that if the Daleks are afraid of The Doctor then he must be rescued so that he can help them defeat the Daleks. This they do and it is then that The Doctor finds out that the guerrillas have brought a bomb made of dalekanium to the 20th Century. The Doctor realises that their planned use of this powerful but unstable explosive means that the guerrillas are caught in a predestination paradox. They will cause the very explosion they went back in time to prevent, and will therefore be creating their own history.

Sergeant Benton

After convincing the guerrilla’s leader of this The Doctor and Jo are allowed to return to the 20th Century. However, they are followed there by the Daleks, who intend to ensure their version of the future is preserved. The Daleks and Ogrons start to attack Auderly House – just as Sir Reginald Styles and the delegates for the peace conference arrive. In the ensuing battle between the Daleks, Ogrons and UNIT, The Brigadier is able to evacuate the delegates.

The Doctor and Jo seek out Shura to try and convince him not to activate the bomb. They eventually discover the injured guerrilla planting the bomb in the cellar under the house and inform Shura that now the house is empty detonating the bomb will be for nothing. However, on hearing that the Daleks are entering the house, he tells The Doctor and Jo to leave so that he can take care of the Daleks. As UNIT fall back to a safe distance Shura detonates the bomb, destroying the house and all the Daleks in it.

With the crisis over The Doctor tells Styles that it is now up to him to make the conference a success. Styles assures The Doctor it will be, because they know what will happen if they fail. The Doctor, nodding at Jo, says that they know too.