42. Fury from the Deep

Good bye Victoria! Hello Sonic Screwdriver! After a season on board he TARDIS, we bid farewell to Victoria. And for the first time, the Doctor uses his sonic screwdriver. This story also has one of the most terrifying monsters ever. Unfortunately we'll never see it because all episodes of this story no longer exist.

http://en.wikipedia.org/wiki/Fury_from_the_Deep