What P7 is about:

In this alternate timeline of the Halo universe, Reach falls in 2541, and along with it, the last of the SPARTAN-IIs. The Master Chief is defeated at the hands of Field Marshall Kano 'Gorramee, who becomes hailed as a holy hero to the Covenant, branded with the name "Demonslayer".He is soon endowed with the Fleet of Rightful Indignation.
Over the next year, something has happened to Kano, and has supposedly taken his fleet on a mission to destroy the next human colony in line for destruction, Rhodes. Unbeknownst to most of the crew and the rest of the Covenant, Kano, Commander Thel 'Vadamee, and a select few subordinates have gone rogue, and plan on locating something said to be on Rhodes that can help liberate his species from the Prophets.
On the human side of things, with the last SPARTAN-II KIA, the Office of Naval Intelligence has begun to rely on the SPARTAN-IIIs, with one group above all.  Known as ROYAL Team, consisting of proteges of SPARTAN A027-Jacob, they are sent to help defend Rhodes while its civilian population is being evacuated.

In Kano's fleet, rumors are beginning to spread about their leader's real intention, sowing substantial discontent in some groups. He is unaware of how serious the situation is, being distracted by confirmation that what he is looking for is indeed on Rhodes. But it appears that the secretive ONI also knows of the item, and has stationed extremely heavy resistance around much of the planet's northern hemisphere, attempting to defend the icy Point Stratton, where it is supposedly located. Soon this becomes a race against time between Royal team and the Demonslayer to reach Point Stratton, and discover a secret that will shape the fate of Rhodes, and the future of the entire galaxy...

(For future reference, P7 will last a total of 25 chapters, along with the prologue, epilogue, and a short intermission in between Act 5 and the Finale. If you want to see more P7 stuff than what's already here, go to [link] .)

UPDATE 1/6/13: ACT 2 of P7 has begun.