Sierra 117 is the second campaign level of Halo 3. It is the first in Halo 3 to feature real gameplay, as opposed to the first level, Arrival, which is purely a cutscene and tutorial.
This is also the first level to feature Cortana Moments.
Upon earning 15,000 points in Campaign Scoring, the player will be awarded with the Guerilla achievement and 10 Gamerscore
After stowing aboard The Prophet's ship in Halo 2, Halo 3 begins with Master Chief escaping from the main ship, and crash landing in the Tanzanian jungle. Having been found by Sergeant Johnson, Sergeant Stacker, some Marines, and the Arbiter Thel 'Vadam, John-117 is escorted through the jungle, not knowing there is trouble ahead.
Johnson has with him two squads of Marines, but, after spotting a Covenant Phantom overhead, he decides to split them in order to attempt to reach an evacuation point at a nearby river.
The Master Chief, Thel 'Vadam and one squad head one way, following a stream leading to the river, while Johnson and the other UNSC forces scale a waterfall, and head another way.
Bravo squad, apparently left behind by Johnson's team to guard the Landing Zone, where Pelicans were to extract the troops, breaks radio silence, and is attacked by a pack of Brutes, lead by a Brute Chieftain.