Lieutenant Cain was a partially Cyberconverted human. He was Colonel Joshua's second in command. On the P'tarr planet, Joshua sent Cain and a party of men to investigate a large box in the jungle. They encountered the Fourth Doctor, Leela and Trok'larr, and despite being ordered not to kill, Cain's best friend Abel killed Trok'larr.
Cain and his men took the Doctor to Joshua. Joshua wanted to exploit the unique vegetation of the planet. After the men were attacked by P'tarr, and Joshua killed himself, Cain promised the Doctor that he and his men would protect the planet and keep it secret. (PROSE: One Bad Apple)