Doctor Harry Sullivan is a commissioned Surgeon-Lieutenant in the Royal Navy, who is attached as medical officer to UNIT, organization where the Doctor acts as scientific adviser. After witnessing the Doctor's third regeneration, the Brigadier tasks Harry with the job of confirming the Doctor's fitness.

Harry incredulously follows the Doctor and Sarah Jane into the TARDIS, and is soon bewildered to find himself along for the ride on several adventures.

Harry is a typical old-fashioned English gentleman, yet also has a can-do spirit that lets him adapt to the incredible reality he has been thrust into. Although he is brave, he can also be a bit clumsy at times, and does cause his share of annoyance for he Doctor. Harry is a likable sort, however, and gets along well with both Sarah Jane and the Doctor.

Harry decides he has had enough of traveling in the TARDIS at the end of Terror of the Zygons, and returns back to London by train. He returns to his posting at UNIT and aids the Doctor again during The Android Invasion.

Although this is the last we see of Harry Sullivan, he is mentioned many years later by Sarah Jane as having continued on as a doctor and having saved many lives with his vaccines.