Meregrass would deliver letters and presents to the prisoners, for money. He sold a program to Constantine Ethelred Gurney which allowed him to fool the Wardens, making them think he was the governor and that governor Lok was a prisoner. He also got a tip-off from a Mantasphid prisoner about the war on Myarr and took advantage of it by selling weaponry.
During delivery to Myarr, he encountered the Tenth Doctor and Martha Jones, who were looking for the data chip he carried. During the battle, the Mantasphids wanted to use the weaponry, which required a code from Meregrass. When they refused to pay him, he stormed off, leaving them to their fate. He was killed by Baltazar who killed Meregrass by shattering his water tank. Baltazar remarked that he dried out very quickly. (TV: The Infinite Quest)