Blimey, you must be new to Dr Who! They've been hunting for the missing stories for years. In the middle of the 1970s the BBC had the idea that it would never re-broadcast most of the old shows it had stored and decided to junk a lot of them to make space. All of the Dr Who shows from the 1960s and a fair number from the early 70s were lost. However, BBC enterprises, which handled overseas sales, had kept film copies of quite a lot of these episodes and these were returned to the main archive when the BBC realised its mistake. Since then, a worldwide search has recovered a few more stories and odd episodes. All the missing episodes are from the black and white period, although a few of the colour episodes are only held in monochrome.
There is a list of the missing episodes here:
http://www.bbc.co.uk/cult/doctorwho/episodeguide/missingepis odes.shtml