Sakuya and his group are heading to Reginleif, an island owned by Shardina.
However, their ship starts to take on water along the way. Sakuya is thrown into the sea from the sinking ship.
When he wakes up, he finds himself washed ashore on a small island called Aquario.
After confirming that Yuri and the others are safe with Lilithea, who has come to meet him, Sakuya joins them at a mansion built on the island.
They are warmly welcomed by the mansion's owner, Luciola, and the waiter/tutor, Ursuna, and the group deepens their friendships.
Meanwhile, a photographer named Harvey shows up at the mansion and begins to act suspiciously...

Just as they uncover the truth behind Harvey's murder, another murder occurs.
Shardina suddenly appears and gives Sakuya a deadline to solve the case.
If it's not solved by 6 AM the next morning, the island will be bombed with missiles.
They head to the crime scene, but Dmitri's body is nowhere to be found.
All that remains is a large pool of blood, a scarf, and the word "Siren" written in blood on the wall.
Sakuya and his team continue their investigation, but more victims appear...