Naoto and K4 head toward Yokokawa as the rain plummets down. Haruka says the track from Usui Bridge No. 3 to Yokokawa should be safe since it's regularly maintained for the opening of the Usui Abt Railway, but the downgrade is steep, they can't slow down, and the narrow tunnels make it difficult to handle curves... They're in a serious pinch when Iwaizumi takes action!