The Triangle Mystery is the twelfth and last installment in the CLUE series. This time it is the Devil’s Triangle – a triangular area out at sea, not far from the shores of Schooner Cove – that sparks a great new mystery for the CLUE gang to solve. During the past century, the Triangle has become infamous for the suspicious shipwrecks and airplane crashes that occur within it just a bit too often. As sunny May rolls around, two scientists arrive at Schooner Cove to study the phenomenon. Cecilia, Leo, Une, and Une’s dog Egon are meanwhile looking to recover some mysterious images from some suspicious characters. Triangles and patterns aren’t only showing up inside the Devil’s Triangle out at sea, as it would happen.

In The Triangle Mystery, the mystery of the priceless Bolette ring is at last solved as well, concluding the storyline that began in book nine of the CLUE series, The Wolfhound Mystery. The philosophical backdrop of the twelfth and final CLUE book is the teachings of Pythagoras: his famous theorem on right triangles and his belief that “all is number”.