Cecilia, Leo, Une and Une’s dog Egon have gone to explore the old Esmeralda mine, the deepest of the three mines on the mountainside behind The Pearl, Cecilia’s father’s guesthouse. The mine has lain empty and unused for years, and deep inside the four friends find traces of the great accident that happened there: black powder and bags filled with old dynamite sticks. But inside the dark mineshafts they also make a more sinister discovery, one that hints at the old mine hiding even bigger secrets: secrets that won’t stand up to the light of day.

The CLUE series’ name comes from the first letter in each protagonist’s name: Cecilia, Leo, Une, and Une’s dog Egon. In each of the series’ books a new mystery and a new ethical question is introduced along with the thoughts of one of the world’s most well-known philosophers. The CLUE books are best selling crime fiction for younger readers aged 9-12.