Simon is a boy who lives underground and digs to make a living. In Simon's world, all humans live underground, surviving at the bare minimum by whatever they can gather. Kamina is a man who aches to advance further and see the surface again that he saw with his father when he was but a child.

His prayers are answered when one day a huge mech plunges through the ceiling into their underground village, creating havoc and panic. Meanwhile, Simon digs up an extraordinary weapon, a small mecha, which Kamina later names Lagann. In the ensuing battle, Kamina, Simon and a huntress called Yoko band together to take down the enemy and emerge to the surface.

From that point onwards, Kamina and Simon only head further and further down the road to their ultimate goal of going beyond the heavens.