Episode 87.

The Doctor is a renegade Time Lord: an eccentric, highly-intelligent scientist from a distant planet. He travels through time and space in the TARDIS, a curious device, larger on the inside than on the outside, which was designed to change its appearance to suit its surroundings. Unfortunately, the Doctor's TARDIS seems to be broken, and always appears as a blue British police box. The Doctor has a soft spot for the planet Earth, and often visits there, either to save it from various alien threats or to whisk a choice few inhabitants away to the distant parts of the galaxy to help him fight evil there. The Doctor has many foes, including Daleks (led by Davros), and The Master, another renegade Time Lord. Time Lord biology enables them to regenerate their bodies, and so both the Doctor and the Master appear evolve over the years...

When the TARDIS lands in on Earth in a quarry, the Doctor and Sarah are caught in a mining explosion. She is found clutching what appears to be a fossilised hand, buried in 150 million-year-old strata. Analysis shows the hand to be silicon-based and inert, but when Sarah begins to act as if possessed, the Doctor suspects that it may still be alive...