The Daleks prepare to launch a deadly plague missile that will kill everyone on Exillon.
The pattern is an electrified trap, but the Doctor is able to negotiate it with his sonic screwdriver. The 7000-volt electrical charge causes only minor damage to the Daleks. The next two tests are of the mind – the Doctor has to use his sonic screwdriver to stop Bellal killing him – and of their sanity – they manage to survive after the Doctor denies the reality of the mind-warping illusions. This opens the final area – the City’s control center, where the Doctor promptly starts tinkering with the City’s “brain”. It responds by triggering the creation of two humanoid “antibodies” to destroy the intruders; the Doctor orders Bellal to let him know when they’re finished while he keeps working.
At the mining camp, Sarah finds Jill Tarrant and the pair hatch a plan to replace the Daleks’ parrinium sacks with sand-filled ones, whilst the real parrinium sacks are moved to the Earth spaceship. As dawn breaks, a Dalek guard finds that Jill has escaped, and promptly self-destructs in remorse.
On the City exterior, Hamilton and Galloway have finally reached the summit and plant one of the two Dalek bombs on the beacon. Galloway, however, insists on keeping the second bomb, pointing out that the Dalek guard at the base won’t be able to see them keeping it and that one bomb will be enough to do the job. They start to descend.
The Doctor has almost finished with his interference with the City’s “brain” when the antibodies attack him and Bellal. Fortunately, the pursuing Daleks arrive and promptly fire on the creatures, causing them to turn their attention to the Daleks. The fight causes a lot of damage in the control room, and that plus the Doctor’s tinkering causes the City to start to die; the Doctor and Bellal escape.
The Dalek bomb goes off, and power is restored. As the Doctor and Bellal escape they join up with the other humans, only to find that the Daleks are not only about to leave, but that they really were behind the plague. As soon as their ship is in orbit, the Daleks will drop a plague bomb to wipe out life on Exxilon and prevent anyone else from coming to the planet to get more parrinium, while the large quantity they mined will allow them to hold the galaxy to ransom.
However, Galloway, who hid on board before the launch while loading the parrinium, sacrifices his life to destroy the Dalek ship with his stolen bomb. Sarah and Jill Tarrant then reveal that they had replaced the Daleks’ parrinium with bags of sand, and that the real stuff is on their ship. Jill Tarrant and Peter Hamilton will now await the arrival of a rescue ship to bring the much-needed plague cure to the afflicted planets.
Everyone turns to look at the City as it melts away, screaming as it is destroyed. The Doctor bemoans the City’s destruction – now the Universe has only six hundred and ninety-nine Wonders.