3x13 - Justice League

Batman, Green Lantern, and Wonder Woman chase Chronos into the future Gotham City of Batman Beyond, just in time to face a battle with a group of Jokerz beside that era's Justice League. The time travelers are taken to the Justice League Unlimited refuge. An older Bruce Wayne reveals the street gang they fought together was enhanced by Chronos who led them to kill the rest of the League of that era. The combined heroes defeat the retooled Jokerz, and Batman traps Chronos in a time loop.