Angel Beats! Heavens Door is a prequel to the anime Angel Beats. It's based on the novel Angel Beats! Track Zero, and tell us the history of Yuri, Hinata and the others before the creation of the SSS and the meeting with Otonashi.
The history begins when Yuri meets Hinata for the first time. So yuri convince in to join SSS as the first member and go search for the god of the after life world.