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 Cybermen archive contains information about Cybermen's encounter with the Fourth Doctor. The episode "Revenge Of The Cybermen" takes place in their future. It's placement suggests that "Revenge Of The Cybermen" took place before "Earthshock", which is the year 2526. According to the Doctor in "The Ark In Space", when he first steps out onto the station, the Nerva Beacon is 29th or 30th century technology. Viewers have speculated that the "Revenge Of The Cybermen" clip is possible because the Cybermen had travelled back in time to prevent the Cyber Wars - which would also explain their more advanced design. There is nothing in the episode itself, however, to support this notion. See more
[regarding the inside of the TARDIS
It's bigger than you think.