Phi Brain: Puzzle of God (2011) : 3x6

Elena, who had just come back from abroad, becomes curious about Raetsel and decides to hold a welcoming party for her to find out more about her. This attracts the interest of Ana, Airi and Tamaki, who all invite Elena to hang out with them, hitting the shops before going to a pool. Meanwhile, Gammon, who becomes irritated that his bounty is lower than the others, is called out by a Master Brain named Colonel Demolitionman, who traps Elena's group, along with Kaito and the others, inside the pool building. This puts pressure on Gammon, who has to solve a set of puzzles from inside a tank before it smashes into the building with everyone inside it. Despite facing up against great odds, Gammon's determination to not lose lets him solve the puzzle and stop the tank in time. Afterwards, Raetsel confronts Enigma, who put her at risk as a reminder of her mission. As Raetsel chooses to break her new ties of friendship and go back to her old self, Melancholy is seen with Raetsel's group.