Greg, an eccentric empty-nester, spends his days so alone, he looks forward to receiving spam calls just to have someone to talk to. One day, just as the subway dips below ground, he receives a mysterious new call, inviting him to get off the train. He follows the caller's instructions, wandering through the abandoned tunnels, and arrives in an underground world full of monsters and myths — the caller is revealed to be a gigantic talking bug, who becomes Greg's guide. Through some sort of miscommunication Greg has been chosen to save the underworld.