The script is, first and foremost, a character-driven story about a soldier named John who was kidnapped or "conscripted" by the UNSC when he was just six years old, and then brutally trained to become an elite Spartan warrior known as Master Chief 117.
The script then takes us through the horrific first contact with the Covenant hordes on the doomed colony world of Harvest, and then climaxes with the spectacular fall of the UNSC forward base on Reach, during which every other Spartan is slaughtered.
The script also gives detailed outlines for the second movie, HALO: RISE OF THE FLOOD, which takes place entirely on the Halo ringworld, and the third and final movie, HALO: BATTLE FOR EARTH, which roughly follows the events of Halo 3, the game.
One cool advantage of this first script is that (like the shark in JAWS) you don't even see the Covenant until halfway through the movie. And because all the creatures are CGI creations, this cuts the budget down dramatically and makes a first Halo movie that much more viable. For Halo fans, it's like the prequel that provides all the answers to questions they've thought about for years. For non Halo fans, it's an exciting action movie that provides a clear, concise introduction to a world five hundred years in the future with relatable characters and a terrifying alien menace.
