Professor Quatermass is resisting the planners of a rocket project known as the Dead Man's Deterrent. Meanwhile excavation is going on at Hobbs Lane, long the site of reported imps and devils.

Following the drilling attempt, a hole has somehow opened up in the wall which allows Quatermass and the others access to the interior chamber. There they find the remains of insect-like aliens resembling giant three-legged locusts, with stubby antennae on their heads giving the impression of horns. As Quatermass and Roney examine the remains, they theorise that the aliens might have come from a nearby planet which was habitable five million years ago — Mars.

Despite his warnings, the media event occurs, and the power cables that string into the craft fully activate it for the first time. Glowing and humming like a living thing, it starts drawing upon this energy source and awakening the ancient racial programming. Those people of London in whom the alien admixture remains strong fall under the ship's influence; they merge into a group mind and begin a telekinetic mass murder of those without the alien genes, an 'ethnic cleansing' of those that the alien race mind considers impure and weak.