- last post: 01.01.0001 12:00 AM PDT
It's all in the 3rd halo book 'First Strike'.
Basically, before the events of halo, before sgt was a sgt, he got into a firefight with some covenant. In the scuffle, he captured a whole box of plasma grenades, which he used to terrifying effect. He got a medal, promotion, and tons of radiation as a side effect for using all those grenades. That caused 'boren's syndrome', which requires extensive medial treatment. The sgt didn't want to wait for the whole recovery process to take place, so he foregoed it.
How does this relate?
Well, as the chief saw in Jenkin's video, Keys, Johnson, and a bunch of other marines were ambushed by the flood. They tried to possess Johnson, but the boren's syndrome made it impossible for the flood to 'interact' with his body, so they just left him. He made it back to some marine's camp, and he, along with few others (a odst marine, a lieutenant of section 3, and warrant officer polaski who flies the pelican that gets em off halo).
At the ending of halo 1, Master Chief hooks up with said dropship, and attemps to make it back to earth.
Now go buy the book and finish the story. It's worth the read.