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.
When Alydon refused to abandon the pacifist life his people had known in the centuries since the neutronic war between his ancestors and those of the Dals, Ian devised the ruse of threatening to take Dyoni to the city and give her to the Daleks for experimentation in exchange for the fluid link needed to make the TARDIS operational again. As he had hoped, Alydon's love and protective feelings for Dyoni moved him to strike Ian as he attempted to abduct Dyoni, thus proving that he and his fellow Thals were in fact willing to fight.
Dyoni participated with her fellow Thals in the attack on the Dalek city. After the Daleks had been defeated, she gave Susan her cloak as a token of thanks and friendship.
From the moment her people had first discovered it in the jungle, Dyoni was intrigued by the "strange blue box" from which the Doctor and his companions had emerged. Her intrigue became even greater after her adventure with these alien visitors had concluded, when she witnessed them enter the box and watched the box fade into thin air. She knelt in the space where the TARDIS had stood a moment before, still puzzling over its mysteries. (TV: The Daleks)