Arrival

Arrival is the tutorial level of Halo 3. It it does not contain actual gameplay, consisting instead of a tutorial (on lower difficulties) and the game's opening cutscene.

Plot
Master Chief crashes in the jungle and is eventually recovered by the Arbiter, and several marines. This level consists of no game-play (save on Easy and Normal, see above)'', only a cinematic cut scene.

In Heroic and Legendary the only point in doing this level is because you cannot get the Campaign Complete on Heroic/Legendary achievements without doing it. Other than that it is completely pointless on the harder difficulties.

this is a continuation of the small film trio landfall.

Arrival/Sierra 117
This is a guide how to complte the first level of Halo 3;done by Spec 0ps Commander.

Walk It Off
Like a beacon, he knows the way.

Follow your team until you come to an opening with a waterfall. Once you reach the entrance into the clearing, a Covenant Phantom appears and drops the first enemies you encounter. Follow the path up to your right, and engage the troops in the cement bunkers to your right.

After you have finished off the first wave, a second wave appears, dropping from a Phantom that appears at the little waterfall in front of you. Kill this wave and cross to the other side of the river via the rocks along the waterfall, and kill the third wave of troops. Note: Try killing the Brute first as this sends the Grunts into panic mode.

Kill the second and third waves, Brutes first, then Grunts.

After dispatching all the troops in the immediate area, follow your team through the cave tunnel marked with a red flare.

Emerging from the cave, you'll come across a horde of sleeping Grunts. As instructed by your squad, take out as many as you can with a melee attack. (You'll kill about five Grunts before they wake up, thirsty for revenge.)

Then, prepare yourself for heavy resistance -- Jackals, Grunts, and Brute Commanders. We recommend a pair of Brute Spikers, as they do considerable damage to even shielded enemies when used in tandem.

You and your squad will keep moving towards the Sgt., entering a small ravine. As you enter the ravine you will see a Brute holding a marine by his throat above you. If you are quick enough, you can kill the Brute and rescue the marine, bolstering your own forces.

With your new teammate in haul, make your way up and around onto the upper area where the brute was, then enter the large cave marked by a red flare. A vision of Cortana flashes on screen. Follow the chain of red flares until you make it to the end of tunnel.

Charlie Foxtrot

You'll find the bodies of marines as you emerge from the cave. Search them and you will find a Battle Rifle.

Now that you have the Battle Rifle, continue forward until the path drops off. Ahead of you by the river are several enemies fighting a pair of Pelicans. Up on high ground, use the Battle Rifle to take them out, then advance to their location on the river.

Approaching the river, you'll see a pair of Banshees swoop in and down the Pelican that Sgt. Johnson is in. Although you've reached the river for evac, your evac has been taken out, resulting in a new objective.

Objective: Find Johnson's crashed Pelican

Kill the enemies fighting the Pelicans, then move into the ravine to meet your allies. With only a few steps into the ravine, you'll receive fire from Jackals with Carbines along the upper walls of the ravine, but they're easily spotted because they'll flash a purple light while aiming at you. Move along the ravine, dispatching the Jackals and other ground forces, until you come upon a point where you see marines fighting. Turns out they were aboard Johnson's Pelican! You've found the crash site

Objective: Find Johnson before he's captured

Fight off the forces attacking the crashed marines, including the ones dropped in by the Phantom, then collect the Sniper Rifle and ammo by the downed Pelican.

With your newly found Sniper Rifle, head back the way you came, towards the cave with a red flare by its entrance surrounded by your troops.

Enter and move through the cave, down its path, until you come to a cliff overlooking an enemy installation. A cut-scene ensues about an imprisoned Johnson, prompting a new objective.

Objective: Rescue Johnson and his squad 

Take out as many troops as you can from your high ground position. Also, several troops sit right below you on the other side of the dock to your left.

Drop down and proceed forward to the bridge that allows you to cross to the other side of the dock. You will have to fight several Brutes on the bridge, including a commander complete with Gravity Hammer. Kill him and take his Hammer.

Use the Hammer to kill the remaining forces standing between you and Sgt. Johnson's cell. Free Johnson, drop the Hammer for a different weapon, and prepare for your new objective.

Objective: Stay alive - Pelican inbound

Two Phatoms will come in and drop off several troops. Finish them off and then join Sgt. Johnson and the remaining marines back on the bridge between the docks. Jump on the Pelican and the mission is over.

At the end of the level. Face the Pelican that arrives. Turn 90 degrees left and jump onto the large ledge. Walk all the way down until you see a thinner ledge on the building in front of you. Jump onto it and follow it all the way to the end where the Skull sits in the shadows.

Trivia

 * The name may be a reference to Halo's predecessor, Marathon, an earlier Bungie game, in which "Arrival" is also the name of the first level.
 * Arrival is the first tutorial level in the Halo series not to take place on a human space vessel orbiting a celestial body. Halo: Combat Evolved's very first level was on the UNSC Pillar of Autumn orbiting the Halo mega structure and Halo 2's first level was set on the UNSC MAC defense platform Cairo in a geosynchronous orbit around Earth.
 * If you look closely at the data pad the one marine carries, you can see an x-ray of Master Chief's helmet revealing his skull.
 * Contrary to what players might think, the Master Chief did not land on Earth using just his MJOLNIR armor as protection. At the other end of the crater is a metallic object, suggesting the Chief ripped it out of the Dreadnought and rode it down to Earth. Also, the metallic object appears to be a balcony or platform of some sort due to the support bracing on one side.
 * Look behind you as soon as you start the level, the above statement can be confirmed that the Master Chief did not use his MJOLNIR armor for the impact protection, rather he used a clear-cut Forerunner door that he used as a shield.
 * The IWHBYD skull does not affect what Johnson says in the level, only the difficulty.
 * As soon as you can move, go forward and press Y about 1 second before the Border fades out. Your Pistol will just randomly appear on the screen.
 * It should be noted that in Arrival, re-entry occurs before sunrise, and in the Landfall miniseries, re-entry occurs after sunrise (though they are in different places).
 * The Landfall miniseries takes place many miles west of the reentry point in Arrival, accounting for the time difference.