[Update] Microsoft is officially not screwing around with this leak. This video was taken down inside of 20 minutes.
This is the opening of the game, not the closing, but we'll give it a spoiler alert anyway, for those who like to bring home a new game, turn off the lights, and watch everything start to finish before jumping right in to play.
The scene opens with Dr. Catherine Halsey, the scientist in the Office of Naval Intelligence who created both the Spartan program and the Mjolnir armor the soldiers wear. She's being interrogated by some agent, not from ONI. Her dialogue justifies her actions in a project with dark origins—basically kidnapping children to begin the project.
The introduction then shifts to the derelict UNSC Forward Unto Dawn, where Cortana reawakens Master Chief. This scene was first alluded to in the game's teaser trailer at E3 2011.
YouTube video uploaded by Madchinima