The Investigator was an Earth Empire official sent to launch enquiries into the death of the Administrator of Solos and the handling of the colony by the Marshal of Solos.
While initially sceptical of the Marshal owing to the Marshal's frequent outbursts of paranoia, he was committed to due process and unwilling to take the Third Doctor solely at his word on the Marshal's crimes. Terrified at the sight of a Solonian mutant, he relented and gave the Marshal his command back, only to find too late that the Marshal had succumbed to megalomania. After the Marshal's death, he appointed Cotton to oversee the Empire's withdrawal from Solos. (TV: The Mutants)
