In 1903, daring American insurance investigator Ben Harris arrives in a secluded Cornish village to uncover the mysterious disappearance of Jill Tregillis' brother, who vanished while treasure hunting. Jill and Ben join forces, uncovering tales of an ancient city submerged beneath the sea, ruled by immortal beings. Their quest takes them deep into treacherous waters and through foreboding caves, where they encounter bizarre phenomena and cryptic messages warning them to turn back. Every step closer to the truth heightens their intrigue and peril.

Intrigues deepen as they stumble upon a hidden passage leading to an astounding underwater civilization that time seemingly forgot. Here, they meet the enigmatic ruler, who harbors a sinister secret tied to the legend of Atlantis. As they delve deeper into the city's secrets, tension mounts and time runs out. Racing against the looming plunge into oblivion, Jill and Ben must outwit ancient guardians and save not just themselves but also unlock the mystery behind the city beneath the sea.