Will You Keep This a Secret? - S1.E10

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.