Rail Wars!

Season 1, Episode 10: Will You Keep This a Secret?

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Prince Bernina of the small Mediterranean kingdom of Atella has arrived in Japan to attend a sister city ceremony in Sapporo, and the K4 members are appointed to guard and guide him on his way there. Naoto and the others are about to board the Hokutosei 3 at Ueno Station when Aoi contacts Naoto and tells him she's pursuing a suspicious man who was watching the platform.