The Japanese adaption of Agatha Christie's famous whodunit "Murder on the Orient Express".

The Kitao family is coping with the loss of the eldest daughter, Chizuko, who was killed in a freak accident the year prior. When the other daughter Mika is suddenly in a dangerous situation, Chizuko returns as a ghost to save her; reunited with her sister, Mika begins spending time with Chizuko again.

Teenage math whiz Kenji Koiso agrees to take a summer job at the Nagano hometown of his crush, Natsuki. When he arrives, he finds that her family have reunited to celebrate the 90th birthday of their matriarch. His job: pretend to be Natsuki's fiancé. Meanwhile, his attempt to solve a mathematical equation causes a parallel world's collision with Earth.