The Investigator was a failed pilot devised by Gerry Anderson and produced by Starkits in 1973.
Synopsis
Two teens, John and Julie, find themselves ⅓ normal size due to the work of the unseen "investigator", and alien being only seen as a green star in the teens' cave base. The investigator has recruited the two teens to make the Earth a better place, for he has seen the evils of humans, and one in perticular is the teens' first mission. Karanti, a rich and greedy man, plans to steal a precious painting from a church, but the teens stop him before he gets away with it.