As the Tardis is travelling the vortex, the Tardis crew are engaged in their own activities. Adric, in his room, alone but when the Doctor stops by he complains he's always being picked on by everyone and considered a joke. He suddenly informs the Doctor he wants to go back to his own people. The Doctor argues it's not possible as they would have to travel through the CVE and E-Space uses negative co-ordinates. Adric is adamant to try and plot a course, but the Doctor is stubborn and refuses. Not put off, Adric decides to do so anyway. Nyssa persuades the Doctor to at least give Adric the chance and see what he comes up with.
The Doctor finally agrees to allow Adric to see what he can come up with. While he's doing that, the Doctor has landed inside a cavern on Earth to go for a walk, Tegan and Nyssa follow him. But unknown to them, a team of palaeontologists were recently down there and killed. It's not long before the Tardis crew run into the security rescue team, who obviously believe the Doctor is responsible. But suddenly they are attacked by robots and the Doctor realizers they are guarding a metallic hatch the palaeontologists were close to when digging. With some persuasion, the Doctor makes the security team realize they are not responsible and offers the Tardis as sanctuary when he discovers the hatch was concealing a bomb.
Using the Tardis, the Doctor traces the source of the bomb to a small freighter on it's way to Earth with a cargo and a tight schedule. Arriving on the freighter, the Doctor asks the security team to stay behind while he and Adric go off to scout about. It's not long before they discover a body and are blamed for the murder. They are brought to meet the Captain of the ship and the Doctor does all he can to illustrate he's not responsible for the death. When things start affecting the ship, the Doctor offers to help. But things turn from bad to worse when the Doctor himself is being watched. Secretly on the ship, a patrol of Cybermen are plotting take control of the ship and have recognized the Tardis.
They Cyber Leader realizes the Doctor is evidently on board and reviews previous incarnations of the Doctor. After realizing none of the men they know were seen, he realizes the Doctor has regenerated once more and it's not long before they figure out who the Doctor is. They can tell by his Timelord arrogance. Not needing to hide their presence any longer, the Cybermen make their move to take over the ship. But the ships security are no match for the Cybermen and are dropping like flies. With the Doctors help, the bridge is secure.
Not wanting to wait any longer, the security team in the Tardis moves out to try and help the Doctor, Tegan joins them. They have learned to concentrate their fire in order to overtake the Cybermen. Upset with the deaths inflicted by the Cybermen, Tegan runs in and grabs a Cyber-Gun and destroys a Cyberman. Meanwhile, the Cybermen have finally broken through the Doctors improved security and have taken control of the ship. They plan to use the ship as an enormous bomb to destroy a conference on Earth, who are plotting to rise against the Cybermen.
More Cybermen but the security are holding their own till Tegan is suddenly captures. She is brought before the Cyber-Leader who illustrates the power he has over the Doctor by threatening her life. He orders the Doctor to take him to the Tardis, leaving Adric on the bridge. With the Cyber-Leader gone, the security from Earth finally makes it to the bridge and helps liberate it form the two Cybermen remaining. The ship is locked on a collision with Earth but Adric sets about unlocking it. He's pressured to evacuate but stubbornly he knows he can solve the problem. Meanwhile on the Tardis, the Doctor uses Adrics gold plated badge against the Cyber-Leader, destroying him. Having solved two of the three locks, Adric succeeds in sending the ship backwards in time 65 million years (merely killing off the dinosaurs), but the Tardis crew watch in horror as it strikes the Earth with Adric still onboard, having been unable to solve the third and final lock when a dying Cyberman destroys the ships computer.
DEPARTURE LOCATION: Freighter crashing into Earth, 65 million years in the past.
REASON FOR DEPARTURE: Being unable to solve a final logic code on a doomed space freighter, Adric dies trying to protect the Earth, which he does but in doing so destroys the dinosaurs.