Callany planned to retire, at which time he would hand the Lankester over to his Chief Mate, De Requin. The Sixth Doctor and Peri Brown were aboard the Lankester, travelling to New Orleans when murders occurred onboard.
While dining, the Doctor and Peri heard the screams of a crew member. Arriving on the scene the crew found Crawford, a sailor, stabbed to death. The Doctor was blamed and arrested. He was released to treat Amy Ivans.
The Doctor went to search for supplies. Callany met him and told him that he had had the Doctor arrested so De Requin would think the Doctor was the suspect. Callany told the Doctor that he really suspected De Requin, always feeling watched by him. The Doctor told Callany that Amy was being poisoned with small doses of mercury. The Lankester ran aground on rocks and the hull was breached. It began to sink. The TARDIS had been stored alongside the breach. It was pulled out when the breach opened.
Meanwhile, De Requin tried to force-feed Peri mercury to kill her. In the following scuffle, Amy's wheelchair was knocked over to reveal that she was a mermaid. De Requin had found her and planned to sell her in New Orleans. He kept her daughter, Galliteya, hidden in a wall compartment.
De Requin shot and killed Callany when he told him there were no lifeboats left. Nereus and his mermaids arrived and killed De Requin and the ship sank. Nereus promised help for the crew who had jumped overboard before the sinking. (AUDIO: Cryptobiosis)