Jacko was a sailor who was shipwrecked near Atlantis. He was captured and put to work in a mine. He and Sean met Jamie McCrimmon and Ben Jackson when the last two were assigned to the mines. The four of them escaped and met with the Second Doctor to aid in his plans to defeat Professor Zaroff.
Jacko and Sean's job was to convince the fish people to go on strike, leaving the Atlanteans without any food. After the Doctor flooded the lower levels of Atlantis, Jacko and Sean fled to the surface, where they saw the TARDIS dematerialise. (TV: The Underwater Menace)