A construction crew digging a new subway tunnel in London excavates a strange metallic missile which the authorities first think is a bomb left over from WWII. They also find the skeletons of several prehistoric, ape-like humans around it. Dr. Mathew Roney is excited about this, particularly the by the fact that they have large brain-cases. When Prof. Bernard Quatermass is called in, he comes to the conclusion that it's really a spaceship. He discovers a hidden compartment inside which contains the quickly deteriorating corpses of large insect-like creatures that look like giant locusts. They are the source of our legends of demons and devils. He has a theory that they are from Mars and that they genetically modified the primitive humans to do their bidding. The Martians couldn't breathe our atmosphere so they created beings they could control that could move about freely. Basically, Quartermass realizes that human civilization owes its intelligence to these long dead arthropods. However, the government doesn't want to believe any of this and does not heed his warnings that the ship still has power. It begins controlling people, reactivating that long dormant part of their brains so that these people start killing those that have too weak a link to their progenitors. Even Quartermass becomes controlled, by a certain extent. It's up to Dr. Roney to stop the alien consciousness from taking over the city.
This is an intense, serious, bleak science-fiction/horror hybrid and my favorite "Quatermass" movie. Highly recommend. Also known as "QUATERMASS AND THE PIT".