Complex sentence: Seok-Woo realized that Su-an was not at her seat, Seok-Woo races with Sang-hwa and Yong-guk to rescue her daughter and the others before being chased back by the horde of infected passengers.
Split into simpler sentences: Seok-Woo realized that Su-an was not at her seat. He races with Sang-hwa and Yong-guk to rescue her daughter and others before being chased back by the horde of infected passengers.