Tuala and Krashen had obtained a couple of time travel devices. They used them to travel to the 20th century, where Krashen would change a person's history for money. However, repeated use of the time machines was slowly killing Tuala and Krashen, and their machines needed repair.
Krashen sent Tuala to the Third Doctor's laboratory to steal some equipment; she took Jo Grant hostage as well, bringing her to the house that was their headquarters. Tuala and Jo had a couple of painful experiences as different versions of themselves came too close together, resulting in temporal energy being released.
Tuala was convinced that the Doctor would be able to help repair the time machines. She tried to convince Krashen to accept the Doctor's help. However, Krashen brought himself and the Doctor to the house when Tuala and Jo were there earlier, and the temporal energy caused both Tuala and Krashen to fade away. (PROSE: Honest Living)