This model of K9 eventually broke down to the point of deactivation during his time alongside Sarah Jane, and sat motionless in an attic for many years because his futuristic inner-workings could not be shared with other humans. Though Mark III was later given the repairs he needed from the Doctor, he remained in poor condition and did not get to enjoy his reactivation for long. In a sacrificial final act, K9 Mark III valiantly gave up his life to eliminate a horde of Krillitanes preying on the human race.