Renton Thurston is a young boy who helps out around his grandfather's mechanic shop; but despite leading a relatively boring life, Renton dreams of traveling the world. However, Renton's life as an average teen comes to a screeching halt when a humanoid mecha called an "LFO" falls down from the sky, only to land right in front of his eyes. From inside the damaged giant, a mysterious girl named Eureka emerges seeking repairs. Renton soon learns that Eureka is a part of the infamous Gekko State, a group of travelers who rebel against the government. He joins them on their travels, but what strange truths about Eureka await him?
Both series tell the tale of humanity's conflict with a strange alien force. In both cases, this alien force attacked the earth at some before the story begins, and sparked off an ongoing strife between humans and the alien attackers. Both series have many things in common, including the use of mecha, the fact that the aliens are extremely alien in both form and mentality, and the fact that the aliens try to take on human forms.