Complex sentence: He also learns there are gaps in his and his schoolmates' memories of what happened at the end of his first school year, including whom he was dating, and tries to find out the truth of what happened.
Split into simpler sentences: He also learns there are gaps in his and his schoolmates' memories of what happened at the end of his first school year, including those of whom he was dating at the time. Yamada then tries to find out the truth of what happened.