Some possible plotholes in the story: Wasn't Cortana stuck in High Charity, in orbit above Halo? And the UNSC ship that crashed into High Charity, was it Miranda's ship overrun by the Flood? And if it wasn't, just how did another UNSC ship reach Delta Halo if Keyes' ship only got there by literally grabbing onto Regret's ship's coattails? And finally, why would detonating the UNSC ship's reactor inside High Charity destroy Delta Halo considering that HC was supposed to be orbiting the Halo. HC would certainly have been destroyed, yes, but not Halo...unless HC landed on Halo. This wasn't made very clear (sometimes the sound of carnage and battle drowned out Cortana's soft dialogue lines and it was hard to keep up with the story). HC landing on Halo would make things many clear, like how it was overrun with the Flood, why would its destruction also destroy Delta Halo and, finally, how Gravemind managed to be inside it at the end (of course, Gravemind might be like the Borg Queen, manifesting itself out of the available Flood 'material' in an infested environment like HC).