Angel Beats!

Season 1, Episode 7: Alive

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Now that Naoi has seemingly joined Battlefront, Yui persuades Otonashi to let his powers of hypnosis help him recover his memories. But Otonashi may not be ready to remember such a tragic, painful past. But there are other issues at hand for the group—such as banding together to capture the mysterious creature from a nearby river known only as the Monster Stream.