During the informal investigation, Conan discovers that the footprints of the armor in the snow were used to cover up the culprit's footprints, and finds that the locked room the son died in was set up. Then Conan detect the unusual point of the Crime Scene where son died. Thus Conan the informal investigation of the case lay down Mouri with his watch to tells everyone that the culprit was the maid, who killed them because they made her father commit suicide.