From the Anime: Gurren Lagann ————————————————

Gurren Lagann takes place in a future where the Spiral King, Lordgenome, rules Earth and forces mankind to live in isolated subterranean villages which have no contact with the surface world or other villages and are under constant threat from earthquakes. Selected villagers called diggers are conscripted to expand their homes deeper underground. Your a meek young digger living in Giha village who is ostracized by your peers, finds solace in your best friend Kamina, an eccentric delinquent who is like an older brother figure to you. Kamina encourages you to join his gang, Team Gurren, to help him achieve his dream of visiting the surface world. One day, you unearths a drill-shaped key called a Core Drill, followed by a small mecha resembling a face called a Gunmen. Soon after, a giant Gunmen crashes through the ceiling and begins attacking the village, followed by a girl named Yoko who attempts to repel the Gunmen. You uses your Core Drill to activate the smaller Gunmen, which Kamina names Lagann, and its drilling-based abilities. You successfully uses it to destroy the larger Gunmen and break through the ceiling, allowing you and Kamina to reach the surface world.

Yoko tells you and Kamina that humans on the surface face daily attacks from Gunmen, with their pilots being Beastmen, humanoid creatures that serve as Lordgenome's army. Kamina hijacks a Gunmen and names it Gurren, combining it with Lagann to form the mecha Gurren Lagann.