Tantei Kibukawa Ryousuke Jikentan 'Shisha no Rakuen'

This case is a few months after the nightmare of "The Kairoukan Murder Case", the second in the series. The story begins with a visit to his friend, Kibukawa, who is in need of a new story.
At the office after a long absence, a beautiful secretary "Haruna Kikukawa", who works at the popular amusement park "Shangri-La," comes to consult.

The request was for security and investigation into a cult-like threatening letter sent to the amusement park and a spooky prank.
However, Hiroshi Kimijima, the president of the company, wants the matter handled confidentially without informing the police.
Despite their disbelief, Ikuo and Itazura head for Shangri-La.

An invitation from paradise is extended to them by a beautiful secretary. A banquet in the name of judgment begins there.
Are the cries of joy echoing in the amusement park that bears the name of an ideal world, or are they cries of delight? Or is it...?