The 'At the Pool' event is one of the Main Events from within the game. The video below shows a scene when all the heroines, acompanied with Chlotz and G.G (who do not appear in this scene) go to a public pool, as the school pool is unavailable for them to use. After the video plays through, it's possible to then go through a bond arc with a chosen Heroine. See Pool Event to see through the bonding choices of the Heroines.
