Krashen and Tuala 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. One such history was Bernard Forbes. However, repeated use of the time machines was slowly killing Krashen and Tuala, and their machines needed repair.
Krashen had Tuala break into UNIT to steal equipment from the Third Doctor's laboratory. He was angry when she failed this mission. He used his time machine on himself and the Doctor, hoping to get the Doctor to fix the machines. However, this was one use too many, and both Krashen and Tuala faded away. (PROSE: Honest Living)