Kingdom Hearts: Chain of Memories is the second game in the Kingdom Hearts series. It is a direct sequel to Kingdom Hearts, and its ending is set about a year before Kingdom Hearts II. The game follows Sora and friends, as they explore the mysterious Castle Oblivion while battling the sinister Organization XIII, a group of antagonists new to the series. The game introduces new characters and plotlines that further expand the Kingdom Hearts universe and set up the premise for Kingdom Hearts II. The game features a new card-based battle system, in a departure from its predecessor's real time combat system.