Harry, the stiff-upper-lip naval surgeon-lieutenant, joined the Doctor during the first adventure of his fourth incarnation (Robot). He was already working for UNIT as their Medical Officer, but encounters with Wirren parasites, Daleks, Cybermen, Sontarans and Vogans satisfied his taste for adventure and at the end of Terror of the Zygons, he decided to remain on Earth

Although he returned a few stories later in The Android Invasion.