Doctor Sullivan is a commissioned Surgeon-Lieutenant in the Royal Navy, who is attached as medical officer to the United Nations Intelligence Taskforce, the military organisation to which the Doctor acts as scientific advisor. He is first mentioned (though not seen) in "Planet of the Spiders", when the Brigadier thinks the Third Doctor has gone into a coma. The Brigadier calls for Doctor Sullivan and asks him to come to the Doctor's laboratory, but tells him not to bother when Sergeant Benton wakes the Doctor by offering him a cup of coffee. In the next serial, "Robot", after the Doctor's third regeneration, Sullivan is called in to attend him, and ends up travelling aboard the TARDIS with the Fourth Doctor and Sarah Jane Smith (played by Elisabeth Sladen) for several subsequent adventures.