Marc Marshall

 was an English teenager. His father was an MP. He was kidnapped by a group of Silurian/Sea Devil hybrids. One of their scientists, Baal, 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 suffered terminal damage by the time the Third Doctor found him. The Doctor tried to get him out of the lair and to the mainland for treatment. However, the strain he experienced during the escape caused Marc's death before that could happen. (PROSE: The Scales of Injustice)