Cessair was wanted for multiple counts of murder and for the theft of the Great Seal of Diplos (which was actually the third segment of the Key to Time). She was able to escape Diplos and take three Ogri from Ogros (which contravened Article 7594 of the Galactic Charter).
She and her servants were eventually captured and transported back to Diplos along with the Megara, sentient justice machines. En route, Cessair managed to escape, taking control of the ship and diverting it to Earth. She went into hiding on the primitive planet, her starship hidden in hyperspace.
Cessair hid on Earth for 4,000 years, adopting a number of identities. These identities included the Cailleach, the Celtic war goddess, Lady Morgana Montcalm and Miss Vivien Fay. Despite the changes in identity, she always controlled the region incorporating the stone circle where the Ogri rested and where, in hyperspace, the starship waited.
In the 1970s, the Doctor came to Earth to find the third segment of the Key to Time. Romana was kidnapped by Cessair and was brought to the starship in hyperspace. The Doctor followed her, but accidentally released the Megara, who sentenced him to death.
After a gruelling trial, the Doctor was able to make the Megara realize Cessair's true identity. Having established her guilt, they sentenced her to perpetual imprisonment and transformed her into an additional stone in the circle.