A futuristic setting in which gladiatorial battles between mechas are the single most popular sport in the world. However, an authoritarian, militant government seized recently control over society and subsequently banned private ownership, developement or construction of mechas, out of fear that such mechas could be used by the growing resistance movement against them. Thus the official mecha battle league came to an end, as the only legal mechas are owned by the military or government-approved corporations.
However, in the unsupervised underbelly of society the thrilling sport of mecha battles continues to thrive: Freelance engineer teams construct mechas from scrap material and components bought on the black market and pit their customised mechas against those of rival teams in underground arena battles hosted by kingpins of the local organised crime. Despite all risks involved hundreds of die-hard mecha fans flock to these illegal battles, and lots of cash is involved in the form of entrance fees, bets and of course victory prizes.
The government rarely gets wind of such battles taking place, but when it does it answers with a swift and brutal crackdown. Arena battles can suddenly turn into a chaotic mess as military mechas and commandos bust into the place to confront the illegal mechas while panicked spectators caught in the crossfire run for their lives.
The Protag and his team, a group of young, talented freelance engineers, dreamed of attaining fame and fortune in the mecha league, but as it got outlawed they saw no other option for themselves but to join the underground battles. Soon they find out that not all teams build and pilot their mechas for money and fame. One of their rival teams turn out to be in fact part of the resistance, using the arena battles as testing ground for their mechas and to look out for potential recruits. Realising the talent of the Protag's team, they try and eventually succeed to convince them to join the resistance.
Eventually the resistance launches a full scale uprising, with the Protag team participating in the climatic battle of the resistance mechas vs the military mechas. The resistance emerges victorious, ousting the authoritarian regime and restoring peace and freedom in their world. The mecha ban gets lifted, and Protag and his team, now lauded war heroes, can at last pursue their dream of becoming the champions of the reestablished official mecha battle league.