DCI Scott Neilson was a police officer with Scotland Yard who spent five years in Special Branch, during which he had dealings with UNIT and heard about the Doctor. By 1978, he was in charge of the Gang Squad and was acquainted with WPC Ann Kelso.
DCI Neilson met the Fourth Doctor after the first attempt on Trenix's life, realising who he was after seeing his TARDIS. He attempted to arrange for a proper safehouse for Trenix after leaving him at Ann's flat and ended up summoning him to the police station, which was soon after attacked by Spandrin and the Sinestrans. DCI Neilson was able to access the Anti-Terrorism Unit, from which the Doctor was able to get them to the roof where Spandrin was killed. (AUDIO: The Sinestran Kill)