Episode No. Title Original airdate
1 "The Unseeing Eye" 21 April 1961
Newspaper reporter, Tom Lambert, decides to reinvestigate the events of a decade earlier when he looked at an incident in which students Clifford Brown and Elizabeth Ryder were invited by their professor to join a researcher working at a radio telescope on Scara, an island off Scotland. They discover a signal and a subsequent disturbance out in the sea and begin to investigate.
2 "The Cylinder" 28 April 1961
The Cylinder is discovered and removed from a sand bank, before being transported to the telescope control room. When the telescope operator is injured while trying to chisel open the discovery, Dr Petrie suggests moving it to his laboratory in London where they can work on it.
3 "The Power" 5 May 1961
A transparent green rod is discovered in The Cylinder which immediately causes damage to the laboratory building. Dr Petrie is invited by Tom Lambert to a former mine cave in Wales in which banging noises were reported and hear the story of a green light being seen by miners previously, linking the situation with that discovered on Scara.
4 "The Voices" 12 May 1961
Dr Petrie continues to gather information on the discovery, including more stories of unusual activity around the world involving a "green light". While alone in the laboratory at night, Elizabeth experiences the power of the rod which begins to control her mind.
5 "The Frozen World" 19 May 1961
Elizabeth is found in the laboratory frozen along with the rest of the room, but still alive. They find a tape recorder in the lab which contains a recording of the situation Elizabeth found herself in. The tape reveals the purpose of the rods, but the tape recorder fails before the entire message can be relayed.
6 "The Unseen" 26 May 1961
A signal is cracked which tells the story of two dominant races on a "tenth planet", with one managing to use the other's technology in an attempt to move to Earth where resources are more abundant. The rod also changes behaviour and begins to move, with Dr Petrie in pursuit, however another cracked signal raises hopes on defeating the rod.