After one of their pilot shoots down an mysterious aircraft over Scotland, the RAF is determined to find out who or what was flying the plane. Meanwhile, a group from SETI is recruited to investigate a possible extraterrestrial message.
An enormous black mountain appears outside of Kirkhaven and begins to absorb surrounding organic material. Reece sends a helicopter to the mountain in order to gather data to stop it. Ultimately, nuclear weapons must be used to stop it.