Killed Again, Mr. Detective - Season 1

High school detective Sakuya Oizuki, along with his assistant Lilithea, boards the luxury cruise ship Queen Illy.
Their current case involves an infidelity investigation. The target is film producer Katsuragi.
While investigating Katsuragi, Sakuya encounters up-and-coming actress Yuriu Haigamine. Upon learning that Sakuya is a detective, Yuriu asks him to help find Katsuragi's cat, and they split up to search.
However, instead of a cat, they find a hanging corpse.
Sakuya examines the clues at the scene, when someone sneaks up behind him...

Sakuya, who was stabbed to death in the throat with a knife, comes back to life thanks to his unique constitution.
The body Sakuya finds is that of Watanoya Nejihiko, the son of the president of Watanoya Confectionery, a famous sweets manufacturer.
Sakuya searches for the person who killed him and Nejihiko. As he continues his investigation, using the scene he saw before he died as a clue, he receives a call from Yuriu saying that she has found the cat she was looking for. That cat holds the key to solving the mystery of the case. Meanwhile, the shadow of the killer looms behind Yuriu as she holds the cat...

Sakuya and Lilithea are invited by Haigamine Yuriu to a private screening of her first starring film.
The screening takes place at the Crimson Theater, a bright red theater with both its exterior and interior decorated in crimson.
During the screening, Sakuya is poisoned to death.
Why was he targeted? After coming back to life, Sakuya searches for clues to the killer in the film that is currently being shown.
Meanwhile, Shardina, one of the internationally wanted criminals known as the "Seven Old Men," seems to be taking an interest in Sakuya...

A water leak at the detective agency forces Sakuya and Lilithia to take refuge at the Kowloon's Hotel.
There, they find Yuri and the film crew shooting their next project, as well as, for some reason, the familiar detective Manroki.
Manroki explains that the director received a threatening letter from someone called "the dog-headed bellboy."
Sakuya and the others decide to patrol the hotel to make sure the film crew is not harmed.
Along the way, Sakuya encounters a suspicious man wearing a dog mask...

The dog-headed bellboy murdered Yuri's manager, Meikuji, and then killed Sakuya.
When Sakuya came back to life, he saw a dragon sculpture in the hotel lobby with Meikuji's head in its mouth.
Four precepts were written behind the reception counter. Two of them were painted over, suggesting the killer was planning a murder based on them.
According to the hotel owner, the dog-headed bellboy had appeared twenty years ago as well...

While concerned about the movements of the "Seven Old Men," Sakuya and Lilithea accept a request to find a ring.
They head to Mizushima Park, the amusement park where the ring is believed to have been lost, and as they search the park, they receive a call from Fido, Britain's top detective.
They try to meet up, but Sakuya is involved in a car accident and they miss each other.
After being taken to the hospital, Sakuya contacts Lilithea and tries to return to Mizushima Park. On the way, a major incident occurs on the Ferris wheel…

Sakuya and his team investigate a brutal murder that occurred on the Ferris wheel at Mizushimaen.
How could someone kill 16 people in 15 sealed gondolas while the Ferris wheel completes one rotation?
A boy stabbed in the back with a knife. A middle-aged couple injected with poison. While the causes of death vary, the victims share a common thread…
Sakuya, cross-referencing the information he received under Fido's instructions, begins to uncover the full picture of the crime. Who is the real culprit…?

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...

Harvey was found impaled on an object in the mansion, his lower body missing.
Who had brutally murdered him? In the midst of the storm, a sound like singing echoed. It reminded everyone of the "Siren," the fairytale creature that Harvey had been pursuing, and they all held their breath. At the same time, Luciola was found unconscious. When
Luciola woke up, she told Sakuya and the others about the attack and said it was the work of a Siren.
After hearing Luciola's story, Sakuya noticed something strange.

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...

In search of clues to the case, Sakuya enters Eliseo's studio with Shardina.
All they find is Eliseo's diary. It contains details about Eliseo before he became a painter, and, surprisingly, a secret related to his immortal body.
As the time limit set by Shardina approaches, Sakuya slowly gets closer to the truth.
The series of incidents are deeply connected to events in Eliseo's past.
The perpetrator of the inexplicable serial murders is...

It was Luciola who killed Dmitri and Ivan.
Luciola is now trying to kill her final target, Katia. Sakuya tries to persuade her, but at the same time, the time limit declared by Shardina arrives. Missiles rain down on the island.
The mansion collapses, and Sakuya and the others fall with the collapsing ground.
Will Sakuya and the others be able to escape from the island...?