Talk about it here.
When the Second Doctor and his companions were about to be sacrificed to the Atlantean goddess Amdo, the Doctor sent a note to Zaroff. Damon gave Zaroff the note and he became intrigued and put a halt to the sacrifice. He put Ben and Jamie to work in the mines, sent Polly to be fitted with plastic gills so she could farm under water and took the Doctor to his laboratory. When the Doctor sabotaged the power system to rescue Polly from an operation, Zaroff became suspicious of him and wanted to keep him close.
Zaroff explained his plan to raise Atlantis from the sea to the Doctor, which involved draining the ocean. He knew his plan would destroy the world, which was, as he saw it, the scientist's ultimate dream of supreme power.
Atlantis's leader, Thous, betrayed the Doctor and Ramo to Zaroff, who ordered them to be sacrificed. They were rescued by the Doctor's companions, and kidnapped Zaroff to stop his plans. Zaroff faked a heart attack, killed Ramo and grabbed Polly to use as a hostage. Polly was rescued by Jamie but Zaroff escaped.
He went to the council chamber and shot Thous and his two guards, claiming the rule of Atlantis for himself. He boldly proclaimed, "Nothing in the world can stop me now!"
Zaroff was very arrogant. He was willing to die in his experiment just to prove himself successful. When he claimed that his experiment was already under way, the Doctor correctly guessed that it hadn't started yet, as Zaroff would want to be the one to push the button. (TV: The Underwater Menace)