Stefan Batak, a Bulgarian who has been spying for Britain, is released in an exchange of agents and C15 are charged with his safety as well as establishing whether his information is genuine. However, he is killed by a poison introduced into his blood stream and the Professionals must trace his killers as well as working out what part his daughter, concert pianist Anna, may have had in the death - if indeed she is his daughter.
—don @ minifie-1