A mineralogist who worked for the Interplanetary Mining Corporation in the 25th Century. An IMC ship, commanded by Captain Dent, arrived on the planet Uxarieus. They found that it already had a group of colonists from Earth present - claiming that the planet was designated for colonisation. Dent claimed that it was earmarked for mining. IMC often disregarded the law, and were capable of resorting to tricks to force colonists to abandon their claims. They used a holographic projector to make the colonists believe that there were large hostile reptiles on the planet, and when this didn't work a mining robot was used, armed with fake reptile claws, to attack an outlying homestead - killing the occupants. An IMC agent was also planted in the colony, to sow seeds of discord and commit sabotage and murder. Caldwell was aware of these activities, but refused to participate in them. He had debts back on Earth, and so Dent had a hold over him.
He found himself sheltering a colonist - Winton - when he was injured and on the run from his colleagues. He pretended to have executed him, so the pursuers would give up.
When Dent forced the colonists to leave the planet, Caldwell warned that their antiquated ship would explode, and tried to do what he could to help. He later helped Jo rescue the Doctor from the underground city belonging to the planet's native species.
Once IMC had been defeated, Caldwell elected to join the colonists and make a new home on the planet.
Played by: Bernard Kay. Appearances: Colony In Space (1971).
- Fourth and final appearance by Kay in the programme. He was the rebel Tyler in The Dalek Invasion of Earth, followed soon after by Saladin in The Crusade. Later, he played Inspector Crossland in The Faceless Ones, as well as the Chameleon leader who had taken on his appearance.