Ia is a precognitive psychic--a powerful one--who can see multiple future timelines and probabilities--and when she sees future disaster, she knows that if she plays everything exactly right, she can DO SOMETHING about it. But she knows she has to join the Terran space Marines first... 
 This is a fun milSF adventure yarn with a kick-ass heroine. In fact, the heroine is so kick-ass that by the time she acquires her Marine nickname "Bloody Mary," I was ready to call her "Bloody Mary Sue" because Johnson has basically written a character with physical superpowers. And of course she is a very powerful psychic AND she can absorb energy in various unconventional ways (this is explained well in the book). To properly use and torture a character like that in fiction, you need insurmountable odds, and it's not clear that Johnson has given Ia enough of a challenge, even though to a normal human being, saving the Galaxy from its alien foes seems like a big enough task. 
 I really enjoyed the book as a whole, but having things made too easy for the heroine knocked it down a star--it could have had 5 stars had Johnson made her character interestingly *flawed* as well.