I'm using alot of different sources to plaster together a plausible plot.
Let's begin shall we.
Okay, in Halo 3, Mendicant Bias sends us to the Legendary Planet.
And in the end of Halo 3 and teaser for Halo 4, the Master Chief is approaching a the Forerunner Shield World.
Onyx is the Shield world in Ghosts of Onyx that Dr. Halsey, Mendez, Blue Team, and 4 or 5 Spartan-III's were stranded in side of. Onyx was torn apart by Sentinels that composed it's inner structure separated shortly after Halsey and the gang departed through the slipspace portal in the center of the planet. Now, say the portal ended up inside a Halo Wars Style Shield World. They start exploring and set up a base camp. After finding their way onto the outer surface which covered in Forerunner Cities. Among the Cities they see a UNSC Research Facility. They link up with the Marines and scientists there.
Now Back to the Chief. He wakes up and sees the Planet and Cortana tells them he is going to crash inside the planet. We have no idea how long he has been asleep. What if it was for over 30 years? He crashes on the inside and finds his way onto the surface where the most of the Gang are still alive except Dr. Halsey and Mendez. All of the Spartans are wearing MK VII.
You see, over 30 or so years, The UNSC forces on the Planet realized they can't get off the planet, and Halsey realized she finally had time. Actual Time for research and development, she spent many years using resources from the UNSC Research Station and collaborating with the other scientists to produce Mjolnir Mk VII Armor.
The Master Chief is equipped with this armor to fight this new enemy.
That's a pretty Cardboard and scotch tape version of what might happen in the intro.