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(I'd like to point out that watching this before watching the series will lead to confusion so make sure to watch everything in order!) This movie is a follow up that picks up where the series left off with a side story. Unlike in the series, this movie focuses on the oldest brother Tenka and his past with the Yamainu Squad. It explains some plot holes from the series (why Tenka and Abe no Sousei have a bad relationship for example), so if your curious about anything from the series I recommend giving this a watch along with the second and third movies.