Harry, at very first much confused by the Doctor, trailed him and his companion Sarah Jane Smith. After the defeat of the robot K1, Harry found himself in the Doctor's TARDIS with them. The three went to Space Station Nerva and were pitted against Wirrn and then by transmat from the space station, travelled to an uninhabited far future Earth in the same time period, where they faced the Sontaran Styre. The transmat next took them to Skaro at the time of the creation of the Daleks. This was for a mission from the Time Lords. Via time ring, they travelled back to Nerva, but to a much earlier year.
Back in the late 20th century, Harry rejoined UNIT to defeat the shape-changing Zygons and their giant, dinosaur-like beast, the Skarasen, in Scotland. The Zygons captured Harry and one assumed his form. He decided to go home to London by train rather than by TARDIS. A short time later, Harry helped the Doctor against the Kraals.
Harry was rather old-fashioned and stereotypically English in his attitudes. He often employed slightly archaic language, referring to Sarah affectionately as "old girl" or "old thing". He was brave, one time saving the Fourth Doctor's life from a bomb on Skaro, and had a "can-do" attitude, adapting well to the many strange situations in which he found himself. He could, however, be quite clumsy and unsubtle, leading the Doctor to declare in a moment of frustration, that "Harry Sullivan is an imbecile!" before passing out from the effects of a rockfall on Voga – caused by Harry! He flirted a bit with Sarah though they kept a strong friendship rather than a romantic one.
