Captain Sinclair was the captain of Lynx, one of the British Imperial Fleet ships landing on the Moon in 1878.
He came to the rescue of Captain Richard Haliwell and his team, including the Fifth Doctor and Turlough, who were being chased by giant spider-like creatures.
He became leader of the group after Haliwell and Emily Boyes-Dennison were taken by a metal claw. He created an alliance with the Phiadoran women and they were helped to travel to the citadel.
Upon saving Captain Haliwell and Emily, he led soldiers to destroy all the servant robots inside the large complex. He then helped to force entry into the large chamber where the Warden was situated. (PROSE: Imperial Moon)