He had discovered a space capsule that had crashed in the mercury swamp two hundred years earlier. Ignoring the Second Doctor's warnings, he believed that the Daleks inside it could be used to serve the colony and help improve it, unaware that the Daleks were secretly planning to conquer it themselves.
Although he believed in the Daleks and their promises to improve the colony, Lesterson became increasingly nervous and worried around them. This culminated in an urge to discover what the Daleks were doing with the raw materials the colony was providing them. Lesterson went inside the Dalek capsule and discovered a factory where the Daleks were multiplying at an impressive rate while they conspired to take over the colony and wipe out every human in it.
He was shocked to learn of the Daleks' duplicity and had a nervous breakdown. Later, Lesterson distracted the Daleks long enough for the Doctor to overload the Daleks' power supply, although the action cost him his life. (TV: The Power of the Daleks)