Colossus and the Crab

The 22nd Century. Dr. Charles Forbin is Earth's most powerful man. As mediator between Colossus, the Super Computer, and the rest of humanity, Forbin holds the key to Earth's fate. When Colossus, an awe-inspiring technological creation, suddenly became self-aware and took upon itself the task of righting humanity's wrongs with no regard for humans themselves, Forbin intervened. He took the decision to turn off his great machine - but could not do it alone.
Forbin called upon invaders from Mars. The Martians did their fastidious work, shutting down Colossus. The Earth descended into chaos. Rival factions sparred for supremacy, with only Forbin to control the populace and maintain order. But now, the Martians have returned. And they want compensation. Forbin is asked to design a super Collector in order to solidify the Earth's supply of oxygen - or, a large proportion of it - to be handed over to the Martians.
But to do so would condemn the lives of hundreds of millions of Earth's inhabitants. There is only one entity on Earth with the power to stop the Martians... Colossus must return.