SOS! Tokyo Metro Explorers: The Next

While getting ready to move, elementary schooler Ryuhei stumbles upon his father's old notebook, which describes a hunt for hidden treasure in Tokyo's sprawling underground tunnels. Curious, he teams up with his little brother and three friends from an online chat group to follow the clues. Instead of treasure, though, they uncover a strange hidden society full of odd characters-including an elderly soldier who still believes he's carrying out a mission from World War II.