The Master, in the guise of Professor Thascalos, has constructed at the Newton Institute in Wootton a device known as TOMTIT - Transmission Of Matter Through Interstitial Time - to gain control over Kronos, a creature from outside time. The creature is summoned but proves to be uncontrollable.

Jon Pertwee
as The Doctor

Katy Manning
as Jo Grant

Roger Delgado
as The Master

Nicholas Courtney
as Brigadier Lethbridge-Stewart

Richard Franklin
as Captain Mike Yates

John Levene
as Sergeant Benton

Ingrid Pitt
as Galleia

Wanda Moore
as Dr. Ruth Ingram

Ian Collier
as Stuart Hyde

John Wyse
as Dr. Percival