Lamune tells the story of a boy named Kenji and his childhood friend Nanami. Although, everyone seems to know that they have a relationship, neither are determined to push it anymore than close friends. The story goes through flashbacks, explaining why they are such close friends and any action doesn't seem to push them apart, but draw them closer.

3rd grade teacher Aoki Daisuke didn't expect the first class he ever taught to be one of the toughest obstacles of his life. After getting off on the wrong foot with the entire class, a moment of kind-heartedness instantly convinces one of his students, Rin Kokonoe, to make the ill-fated Aoki-sensei her lover. But what exactly are Rin's intentions for wanting to actively seduce Aoki-sensei, and will Aoki-sensei be able to help her to deal with them?

In the year 2010 AD, Hyper Hurricanes born concurrently all over the world caused severe damage. Five billion people lost their lives, decreasing the world's population to 3 billion. Existing economies and military were wiped out. Countries were frozen under solid ice, plains sank beneath seas, the world was changed dramatically. Human civilization enters a chaotic era. Poverty flourished outside of these economic zones and slums were widespread. The ruling class called themselves Logos and maintained their position by force of arms. They call the poor Revenus, who are exiled to live outside the special economic areas. Time has passed since then. Katsuragi Jo and Tsurasawa Jin, escape from Phantom, a force organized to watch Revenus and to suppress renegade elements of the Logos, taking with them a mysterious girl, Nobuto Sana. There are many who are interested in her, all with their own reasons.

In California's Center City, shy but kindhearted Joseph Carter "Joey" Jones lives with his grandmother, working in a coffee shop. Though his family is financially burdened and he is bullied at school, he remains cheerful. A robotics enthusiast, he wants a top-of-the-line toy robot called a Heybo more than anything. One day Joey and his friend Simon "Psy" Kaina come across a destroyed Heybo discarded by a group of rich kids. He takes it home and repairs it himself, excitedly naming it Heroman. However, one night Heroman is struck by lightning and comes to life, growing several times its size in the process. That same night Center City is attacked by aliens called Skrugg whose attention was brought to Earth after Joey's acquaintance Professor Matthew Denton made attempts to contact them. Along with Joey, Heroman quickly heads to the scene of the attack, where he finds people in trouble, including his friend Lina Davis and her father. Using an arm-mounted controller, Joey is able to command Heroman to use superpowers to fight off the Skrugg and rescue civilians. As the aliens continue their invasion of Earth, the duo are the only ones separating humanity and destruction. [Written by MAL Rewrite]