With the Doctor cornered in the underground caves by androids, Adric ventures outside of the TARDIS and manages to destroy one of them. They get control of the situation and the Doctor manages to open the door they've located - but in doing so, he's activated a bomb. They set off to find who is behind it all and arrive on board a spaceship on its way to Earth. Unknown to them as yet is that it's all down to the Cybermen, who are determined to destroy the Earth.
Did You Know?
The reason for the Third Doctor's omission from the flashback clips was that he never encountered the Cybermen during his original tenure. In relation to this story, the Cybermen's encounter with the Sixth Doctor, (Attack of the Cybermen) in 1985 would have also been part of their chronological past as would be their 1988 encounter with the Seventh Doctor in Silver Nemesis. Neither were seen as part of the flashback clips. The real world reason for this, of course, was that although these events would have happened already in the timeline, these serials (with their respective Doctors) had not yet been created for the show. See more
When the Doctor and Adric exit the TARDIS, the normal inside of the Police Public Call Box can be seen. See more
A Time Lord. But they're forbidden to interfere.
This one calls himself the Doctor, and does nothing else but interfere.