At the end of Part 3, Ian became the first Doctor’s companion to hide inside a Dalek’s armor so that he, the Doctor, Susan, and Barbara can escape from their cell far underground in the Dalek city and make their way back to the surface without being stopped by the Daleks rolling around all over the place.
The foursome have a few close calls along the way, including an encounter with one Dalek who offers to help Dalek Ian escort the prisoners. It’s due to this conscientious Dalek making a report that the others catch on, and the Daleks attempt to keep the escapees from getting to the lift and going up to the surface–over 100 levels. Now, finally, they’re all “Exterminate! Exterminate!” like proper Daleks. No more pretending to be nice and bringing people lunch on a tray.
Meanwhile, the Thals who received the Daleks message have arrived in the city to get the food they were promised and to settle some sort of peaceful agreement with their old enemies.
The Doctor et al see this group of Thals from a distance just as they return to the surface; they try to shout a warning, but are behind a thick window and too far away for the Thals to hear them. Actually, the Doctor doesn’t want to interfere–he says that matters between the Thals and Daleks are none of their business.
Ian, now out of his Dalek disguise, says that he’ll go try to stop the Thals before they walk into a trap, while the Doctor, Susan, and Barbara run back to the safety of the Tardis.
Ian does reach the Thals in time, but hides around a corner and watches while the Thals wait.
It’s only just before the Daleks charge in that he shouts “It’s a trap!”
That was helpful, Ian. The time to warn people is before the Daleks surround them.
There’s a battle. I noted back when Ian was zapped in the legs in Part 1 that when the Daleks shoot someone, their ray is shown as a photo-negative image of the person being shot. That would remain a constant re Dalek weaponry; in the newer series, we would also get a flash of the victim’s skeleton before they dropped, but the 1963/64 BBC special effects weren’t up to that kind of thing yet, and it would certainly have been considered too grisly and disturbing for the children watching the show. The Dalek “hand” glimpsed in the previous episode drew complaints.
Alydon’s brother, who was the Thal leader, is killed during the ambush. Ian retreats with Alydon and the other survivors.
Back at the Tardis, the Doctor, Barbara, and Susan have met up with the rest of the Thals and everybody’s friends. The Thals have been showing the Doctor some of their ancient records, including star maps that might help him figure out where they are in relation to Earth.
Among the historical documents is a drawing a Thal warrior dressed for battle, showing what these people used to be like before their wars led them to make the planet nearly uninhabitable. The Doctor offers the improbable idea that the Thals mutated out of humanoid form, and back again, in 500 years.
There’s also an old drawing of Dalek pre-mutation, but we don’t get a look at that. (I know from Genesis of the Daleks that they were once humanoid as well and tended to dress like Nazis. In Genesis, we also learn that it was the Thals who first launched their bombs… although Davros was eager for them to do so so he could send over his very first Dalek mutations in retaliation. But that story aired nearly 10 years later )
The storyline could end here, with the Doctor and his companions at the Tardis. The Thals know that they can’t trust the Daleks and may have to resume their old warrior ways to survive. In fact, the Doctor is ready to go, and “leave well alone” on Skaro.
But it’s just then that Ian realizes that the Daleks took the fluid link from him when they were captured. They’re not going anywhere without that crucial component of the Tardis… except back to the Dalek city to retrieve it.