Walkthrough
After your traditional first mission cutscene about Ghost explaining the area you are going to, an area right by the Golden Age city of Freehold, you touch down in the The Barrens. Ghost will explain that you cannot simply get your Sparrow as the Cabal have a lock on orbital communication. To obtain the Sparrow, the Guardian has to break into one of the Cabal's bunkers and disable the system.
Strangely, you will not encounter enemies right away. Use Ghost to open a side door and sneak into the Bunker. Making a U-turn, Ghost will point out a terminal where he can access the system, he will talk about not needing an access key, and setting off an alarm, opening the front door and causing two Harvester drop-ships to fly in, deploying two squads of a Cabal division known as the Sand Eaters. The first Harvester will drop a random assortment of Cabal, and the second will drop in about eight Legionaries. Note: these eight Legionaries will be Majors on Heroic mode. After dealing with the Cabal, you will have access to you Sparrow.
Ghost will say some dialogue about another Cabal outpost, and direct you toward it. You will pass by Firebase Rubicon, and Ghost will mention the Cabal are at war with the Vex here. He will then mention that the Cabal might know something about the Gate, and direct you to another terminal in the Trenchworks. After fighting through some Cabal and accessing the terminal, Ghost will tell you to take out their leader, who happens to be Bracus Tho'ourg, a powerful Major Centurion. After taking him out, you will get a key. Ghost will then tell you to head for the Gate.
Upon leaving the Trenchworks and entering the Valley of the Kings, the first thing you will see on your left is the giant, looming but inactive Gate to the Black Garden, guarded by a few Vex. But instead of going in to the Gate, you go up to another outpost to the right and into a lookout on top of a cliff. Scanning the terminal reveals the Cabal have actually broken the Vex "cognition mesh", possibly a firewall. After looking at what's inside, Ghost reveals the Cabal don't even know what the Gate is, but if the Guardian is going to charge the Eye, they have to go through the Cabal.