With Kaito's group stuck between flights, Cubic and Ana go ahead of them to Rio de Janeiro, where they meet a woman named Regina Maria Vasco. Taking them to a hidden cave under the pretense of helping them find Raetsel, Regina reveals herself to be a Master Brain and traps Cubic and Ana in a Fools' Puzzle, in which they must travel via canoe across several containers whilst making sure they end up with equal volumes. Cubic makes some progress with his calculations thanks to Ana, but Regina keeps interfering with their progress...