While staying in Smallmarshes, he was kidnapped by a group of Silurian/Sea Devil hybrids. One of their scientists, Baal D'jo, experimented on his human DNA in their lair, hoping to discover a way to counteract the sterility and shortened lifespan caused by their hybrid nature. Marc had suffered terminal damage by the time the Third Doctor found him.
Baal allowed the Doctor to take Marc back to the mainland for treatment and let his sister Tahni go with them. When they reached a beach, Marshall, the Doctor and Tahni were captured by security guards working for the Pale Man and taken to the Vault. The strain of the escape took its toll on Marc and he later died when he tried to release Tahni. (PROSE: The Scales of Injustice)