Seven members of the Kyoto Mystery Club decide to spend a week on a deserted island where a quadruple murder took place. Meanwhile, on the mainland, other members of the club begin receiving threatening letters from a dead man.
The first student is murdered on the island prompting the others to instigate a search. Meanwhile, on the mainland, Conan teams up with the mysterious Mr Shimada and begins to suspect that Seiji Nakamura faked his murder.
As the body count on the island rises, Shimada and Conan look into the death of Seiji Nakamura's daughter and the rumours of an affair between his brother and his wife.
A fire on the island seems to put an end to the mystery of the Decagon House but as Shimada and Conan put the final pieces of the puzzle together the solution which reveals itself is much more shocking than anyone could have guessed.
