In 2525, as mankind has begun to colonise space, a group of cadets are training to fight against human insurrectionists. One of these cadets, Thomas Lasky, has doubts about his abilities as a soldier and his convictions for this war. Whilst he struggles with himself, the planet is invaded by an unknown alien race. Reeling under the assault, Lasky and his squad mates are rescued by John-117, one of the UNSC's legendary SPARTAN-II super-soldiers. John must inspire Lasky to fulfill his potential as a soldier and a leader to fight against an enemy deadlier than any that humanity has faced before.