Complex sentence: Gary and Rhonda volunteer to stay and help rescue any remaining survivors before leaving the city by another mean and Nick is about to take off with the others, when he overhears on the radio that Hemlock is planning to create a biological weapon with the zombies.
Split into simpler sentences: Gary and Rhonda volunteer to stay and help rescue any remaining survivors before leaving the city. Meanwhile as Nick is about to take off with the others, he overhears on the radio that Hemlock is planning to create a biological weapon with the zombies.