Under the command of Admiral Sheringham, the Navy has been dumping into the sea containers of an experimental nerve gas which proved to be far too dangerous to be even used. But SHADO discovers that the Admiral's secretary, Sarah Bosanquet, has been subverted by the aliens through a powerful telescope with which she watches the skies. Either they must stop her giving away too much information, or it may be too late to prevent UFOs from finding the containers and releasing the gas - thereby destroying every form of life on Earth!