Ex-Navy Surgeon Lieutenant Harry Sullivan was UNIT's medical officer, who first encountered the Doctor just after his regeneration, and ended up unceremoniously bundled into a cupboard! He later showed his adventurous streak in going under cover to infiltrate Think Tank, and then inadvertently ended up travelling with the Doctor and Sarah when he touched the dematerialisation switch in the TARDIS.

The Doctor - though frequently putting him down - admired his additional companion, and he proved his loyalty and bravery many times in their travels, especially in regard to Sarah, who he was quite fond of (though not of his nickname for her, 'old girl'!). However, after his experiences of being under the influence of the Zygons, he opted to stay on Earth rather than continue to travel with the Doctor and Sarah, though he did get to meet them again during the Kraals' attempts to conquer Earth through android duplicates, including one of himself.