Postman (The Sooner the Better)

From Tardis Wiki, the free Doctor Who reference

An assassin disguised as a postman came to Earth to find Walter Fisher. He encountered Paul, who accidentally gave him a letter for Fisher that had been dropped by the Fourth Doctor. The postman killed Fisher before the Doctor and Leela could stop him, but they captured him and brought him to his home planet to face trial. (PROSE: The Sooner the Better)