Complex sentence: The story focuses on a cold and distant 17-year-old girl, "Nanaka Kirisato" who was involved in an accident that caused her mind to forget everything until the time when she was 6 years old.
Split into simpler sentences: The story focuses on a cold and distant 17-year-old girl, "Nanaka Kirisato". But Nanaka has an accident that causes her mind and memories to be reverted back to the time when she was 6 years old.