In bustling New York City, beneath the chaos, lies the subway. A family of mice lives in the tunnels, including curious young Mark. One day, Mark's father gets news that Grandma is ill and sends Mark on his first solo trip. Ignoring advice, Mark takes a shortcut through the ticket gate, prompting a human chase. After escaping, he reaches Grandma's station but must cross dangerous tracks. Despite a train delay, Mark steps onto the tracks. A sudden roar and blackout follow. The screen displays: "No progress, no security," highlighting the subway's dangers without safety improvements.