Description
Around the turn of the century, American mining engineer Ben Harris is working on the Cornish coast in England when he finds a body washed up on the beach.
Ben makes inquiries at the nearby hotel. While talking to the hotelier's daughter, fellow American Jill Tregellis, and an eccentric artist, Harold Tufnell-Jones, a mysterious intruder appears but disappears.
Later that night Jill is kidnapped by gill men. Ben, Harold and Harold's chicken follow the trail through a secret door into the caves under the house where they are sucked into a pool.
They emerge in a cavernous city on the ocean floor. The city was built by a race of ancients who survive only as a breed of gill men. The city is now inhabited by a group of smugglers led by the cruel and tyrannical Captain who hid down there in 1803 and due to the strange mixture of oxygen have not aged in over a century. However, the volcano that powers the city has become unstable. The Captain now imprisons them until Ben can come up with a means of maintaining it.
Dan, one of the Captain's men who wishes to leave, offers to help the two escape, provided they use their influence to secure him a full pardon for his past crimes of smuggling. The Captain finds out and reveals that because of the gas they've breathed for so long, exposure to sunlight would cause them to age rapidly and die. Dan is then sent to the surface as a means of execution while Ben and Harold are granted an audience with Jill. Whereupon they meet Rev. Jonathan Ives, who had vanished several decades ago from the surface.
The Captain is shown to be under the delusion that Jill is his deceased wife Beatrice, who he believes has returned to him.
Realising that Ben and Harold are untrustworthy, the Captain decides to allow the Gill Men to sacrifice the two as a means to appease the volcano's wrath. While they await this, Jill and Ives free them and Ives instructs them all on how to escape the city. The three make it to the airlock and trek across the ocean floor to a cave, containing a tunnel that leads to the surface.
The Captain and his men pursue them there, but frequent volcanic eruptions cause rockfalls that bury him and his men. Ben and his friends decide to return to the sea and attempt to reach shore on foot. The Captain meanwhile digs himself free and follows the tunnel to the surface, where the sunlight does indeed age him to death.
Ben and the rest make it to shore and watch as the volcano erupts, finally destroying the city.
