Complex sentence: The story begins when, Natsuki Enomoto confesses to her childhood friend Yū Setoguchi, and from embarrassment, denies her love for him calling it a "practice confession".
Split into simpler sentences: The story begins when Natsuki Enomoto confesses to her childhood friend Yū Setoguchi. After the opening, an embarrassed Natsuki denies her love for Yu and calls it a "practice confession".