h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I've always thought this story deserves a mention when the Sontarans come up. They do not themselves appear, of course, but one of their hated enemies, the Rutans, does. They are revealed to be shape-shifters, tentacled blobs in their raw form, and can kill by their own electrical bodies.

The Rutan in question terrorised a lighthouse on an island called Fang Rock in the early years of the twentieth century. It actually succeeded in killing everyone on the Rock, other than the doctor and his assistant Leela. In the end, the Doctor destroys the Rutan with a morter bomb, and uses a diamond and the lighthouse itself as a lazer to destroy the mother ship, before leaving behind a load of dead bodies and an unsolvable mystery.

A fave of mine too. One of the most claustrophobic, most realistic and atmospheric sets in the series' history.