Gemstone, the Mermaid's Scale, appears in a secret, underground auction. The queen of the underworld coerces Lupin into stealing the rock for her. Quickly discovering not only that the stone is a forgery, but that the real Mermaid's Scale is part of an unsolved riddle that leads to transcending mortality!

Learning that island of Delos has been taken over by Neuroi, Charlotte E. Yeager and Francesca Lucchini are teamed up with Hanna-Justina Marseille and her partner Raisa Pöttgen to draw out the Neuroi hiding within it. Francesca, who has strong family ties with Delos, objects to their superior's plan to destroy the island, so they work out a new plan to have Hanna and Raisa draw out the Neuroi while Charlotte and Francesca drop bombs on it. When this plan ends up failing, Francesca tries to attack on but is inevitably forced to retreat. The next day, before the bombardment on the island is due to take place, Charlotte, unwilling to let Francesca down, decides to make use of an abandoned ship's winch to pull the Neuroi out of its hiding place, allowing it to be destroyed without any damage to the island. Afterwards, Francesca takes Charlotte and the others on a tour of Delos and its artifacts. (Source: Wikipedia)