The Kingdom Hearts video game series, developed by Square Enix in collaboration with Disney, takes place in the Disney universe with numerous self-contained worlds based on intellectual properties from both companies. Many of these worlds are based on animated Disney movies, though Kingdom Hearts II introduced worlds based on live-action Disney films as well. In addition to the Disney worlds, a number of original worlds appear over the course of the series. The series centers on the main character Sora's search for his friends and his encounters with Disney and Final Fantasy characters on their worlds. The first game, Kingdom Hearts, takes him through each world to lock their keyholes and prevent the Heartless from destroying them. The sequel Kingdom Hearts: Chain of Memories takes place in Castle Oblivion, where he visits memory-based simulations of many of these worlds that are generated on-the-fly as he travels through them. In Kingdom Hearts II, Sora helps the residents of these worlds again in search of his friend Riku. The Kingdom Hearts games have been both critically acclaimed and commercially successful and the design of the worlds has been praised for its faithfulness to the source material.