Sometime in the 22nd century, humans have fully terraformed and colonized Venus and Mars. The Neosapiens, an artificially engineered race created to be slaves for the Terrans, have turned against their masters and formed the Neosapien Order/Empire/Commonwealth. Under the rule of the calculating Governor General Phaeton, the Neosapiens have launched an invasion of Earth and Venus. The ExoFleet, the Homeworlds' space navy (in particular, Able Squad, a group of Exo-Frame pilots led by Lieutenant J.T. Marsh), fight against the invasion in an interplanetary war to protect the Terrans.
