Ep. 1: Welcome to the Fireworks Festival Kiss!

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While Yuuko, Natsuki and Riko are hanging out at the summer festival, she and Natsuki try to encourage Riko to share a kiss with Goto. Later, they mistakenly see that Goto is about to kiss a women other then Riko, (who is in fact, his sister) and go to beat him up. (Source: Hibike! Euphonium Wiki)