Tom Dekker was a human detective on Earth. In Chicago in 1929, he helped the Seventh Doctor and Ace against Agonal and the resurrected Three Who Rule, shooting the three vampires in the hearts before they could be fully reborn. The Doctor erased parts of his memory concerning these events. (PROSE: Blood Harvest)
He later emigrated to the United Kingdom to head an overseas branch of the Pinkerton Agency. In December 1936, he met the Doctor's prior incarnation in London but remained ignorant of the two Doctors' connection to each other. He told the Sixth Doctor that he had known another man who had called himself "the Doctor" who had run a speakeasy in Chicago. Peri Brown suggested to the Doctor that the man may have been his future self but he dismissed the idea.