Set against the backdrop of the late 19th century, this gripping tale follows the iconic duo, Sherlock Holmes and Dr. Watson, as they navigate political intrigue, international espionage, and murder on the enigmatic Isle of Garth. Tasked with receiving an annual token of allegiance—a pound of butter—Holmes quickly uncovers a sinister plot tied to Garth's secret underwater caverns, a potential haven for submarines poised to threaten England's control of the Channel.

With the island's ruler dead under suspicious circumstances and an authoritarian housekeeper pulling the strings of her young successor, Holmes and Watson race against time to unveil a chilling conspiracy involving German spies, betrayal, and high treason. The story crescendos into a deadly game of cat-and-mouse in the island's subterranean labyrinth, where Holmes' wit is matched against an adversary's cunning ambition.