After analyzing all the data, Quatermass (John Robinson), his daughter Paula (Monica Grey) and Dr. Pugh (Hugh Griffith) come up with their theory concerning what is happening. They have deduced that when the asteroid reaches its nearest point, about 380,000 miles from Earth. It will then release more meteorite objects and move away. After 40 hours the cycle starts all over again.
The meteorite objects are containers that house an alien substance in an ammonia dominated atmosphere, along with methane and hydrogen. When the meteorites crack open the ammonia and the alien entity is released. If there is a person nearby the entity will inhabit the person and take over the person’s will. Quatermass also believes they are a group consciousness. What one learns, they all learn. The most logical place for the entities to originate from is Saturn or one of its moons. Quatermass also believes that the food plant is, in reality, an Earth base that the aliens are building. They are creating an alien atmosphere so they can survive on Earth.
Quatermass decides to ready the Quatermass II rocket in case they need it to destroy the asteroid. The first Quatermass rocket had a prototype rocket motor that exploded when it was tested. The second rocket has the same flaw, which brought Quatermass’ moon project to a standstill. Now that flaw may help them against an alien invasion. The Quatermass II is, in essence, a nuclear bomb.
Quatermass sends Fowler (Austin Trevor) to steal information about the other alien plants, but he ends up triggering one of the aliens’ new ammonia containers and is overcome.
In the meantime, Quatermass contacts journalist Hugh Conrad (Roger Delgado). He tells him everything that is happening and convinces him to go to the prefab town and gather more information. They meet with resistance from the townspeople until alien asteroids start dropping everywhere. Conrad becomes infected but manages to call his paper and give them the story before he is overcome. The townspeople now believe they are being invaded and head, pitchforks in hand, to the plant. As this is all taking place Quatermass is sneaking into the plant and discovering something horrible.
The narration at the beginning of each episode was done by Nigel Kneale.