The mutated Noah has now fully transformed into a Wirrn adult and intends to allow the Wirrn to devour the sleeping humans, absorbing their memories and knowledge.
The Doctor, Harry and Sarah find Earth is a desolate wilderness. They meet a group of stranded human colonists who are being hunted by a robot and experimented upon by its master.
Sarah has been caught by the Sontaran, Field Major Styre, who is conducting cruel experiments on the captured humans, paving the way for a full-scale invasion.
The Time Lords send the Doctor, Sarah and Harry to Skaro at a point in time shortly before the creation of the Daleks. The Doctor's mission is to destroy his greatest enemy.
The Doctor finds that some of Davros's fellow Kaleds are unhappy with the development of the Daleks. Sarah is forced to load a Thal war rocket with radioactive explosives.
The Doctor informs the Kaled government of Davros's experiments and they move to shut down the bunker. But Davros is one step ahead and prepares to take drastic action.
When the Kaled dome is destroyed by the Thals' rocket, the war appears to be over and the Thals rejoice at their victory. But then, the Daleks appear and a massacre begins.
With Harry and Sarah his prisoners, Davros interrogates the Doctor, forcing him to reveal the future of the Daleks so that he can learn from their defeats to make them stronger.
The Kaled scientific elite move to prevent the development of the Daleks while the Doctor prepares to destroy his old enemies forever. But is it already too late?
The Doctor, Sarah and Harry return to Nerva, but at an earlier point in time. The crew are dying and blame a space plague, but the Doctor suspects a survivor may be responsible.
Sarah is the latest victim of the 'space plague'. But the poison can be removed from her body by transmatting her to the surface of the mysterious planetoid, Voga.
The Cybermen who are intent on destroying Voga. The Doctor and two of the Nerva crew become walking bombs which they Cybermen will detonate once they reach their target.
Their plan to blow up Voga thwarted, the Cybermen intend to crash Nerva into it. Vorus is almost ready to launch his rocket at Nerva, but Sarah is still trapped on board.
UNIT are called in when top secret plans and valuable equipment is stolen from high security establishments. The newly regenerated Doctor suggests that the thief may not be human.
Using the deadly disintegrator weapon, the robot steals the destructor codes. Harry Sullivan is sent to infiltrate Think Tank to discover the truth.
Sealed inside an underground bunker, Miss Winters issues her demands to the world's leaders, threatening a devastating nuclear strike if her demands are not met.
The Doctor races to avert a nuclear holocaust. The robot, distraught over killing its creator, takes Sarah hostage and goes on a rampage.
The TARDIS arrives in the far future on the Ark space station where mankind's survivors are in hibernation. Sarah is accidentally placed in suspended animation.
When the Doctor restores the power to the Ark, the humans begin to revive. But the station has been invaded by the giant insectoid Wirrn.
The Doctor subjects himself to a dangerous experiment so that he can access the memories of the dead Wirrn Queen. Elsewhere, the larvae Wirrn are pupating into their adult form.