Complex sentence: On the day he returns to Japan from Indonesia at the airport, Yusuke's life changes as he returns with a mask for Sakurako Sawatari, who assisted in Professor Natsume's archaeological finding of the Tribe and a mummified corpse.
Split into simpler sentences: On the day he returns to Japan from Indonesia with a mask for Sakurako Sawatari. Sawatari had assisted in Professor Natsume's archaeological finding of the Tribe and a mummified corpse.