The third segment of the Key to Time is traced to present-day Earth, but the Doctor finds that while the tracer leads him to the Nine Travellers, a circle of standing stones on Boscowan Moor in Damnonium, the key does not appear to be there. He meets up with elderly archaeologist Professor Amelia Rumford and her assistant Miss Vivien Fay, who are surveying the site, and discovers that the circle appears to have had a variable number of stones through the years.

The Doctor decides to visit Leonard de Vries, supposedly the leader of the British Institute for Druidic Studies (BIDS) which worships at the circle. He is knocked unconscious whilst at the Hall (de Vries's home) and is subsequently saved by Professor Rumford from being the Druids' next sacrifice. He then returns to the house to speak again with de Vries only to find that the latter has been killed by one of the stones from the circle, in truth a life form from the planet Ogros in Tau Ceti called an Ogri, which lives on blood.

Miss Fay turns out to be the Cailleach, an ancient being worshipped by the Druids, and she transports Romana to a spaceship hanging in hyperspace at the same spatial coordinates as the stone circle. The Doctor builds a gun-like contraption which transports him to the ship as well. There he unwittingly releases the Megara, two justice machines which have been trapped for centuries, and they put him on trial for breaking the seal on their compartment.

The Doctor discovers that the Megara's mission was to recapture and try Cessair of Diplos for murder and the theft of the seal of Diplos. He tricks them into knocking Miss Fay unconscious (she had arrived on the ship to gloat), and when they read her mind to determine if she is injured, they discover that she is Cessair.

The machines try her and sentence her to be turned into a stone in the circle on Earth. Before the sentence is carried out the Doctor snatches from around her neck a necklace - the third segment.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}