Dyoni was a member of the Thal party led by Temmosus which journeyed to the Dalek City in hopes of finding food. She was in love with the young Thal Alydon, and hoped to be married to him once their expedition was successfully completed. For this reason, she was initially jealous of Alydon's friendship with Susan. Dyoni showed the First Doctor the historical records of her people. From this he learned of the origins of the Daleks.