Gravkrom-Vey

 was an actor. To prove his acting ability, he needed testimonials from several individuals, so he conned them into believing that he was a Manicoll who was trying to preserve the last of the Monkrah. The Fourth Doctor and Oliver Day were among the people he tried to con, and though the Doctor saw through his ruse, he and Oliver signed the testimonial anyway. The Doctor then stranded Gravkrom-Vey on the Manicoll homeworld to obtain the last signature he needed. (PROSE: Plight of the Monkrah)