Roxas and Xion embark on their first co-mission together under Axel's plan. While in Agrabah, they save Abu from the Heartless, and he drops a gem while running away in fear. The gem grants them deeper access to the cave, where they once again stumble upon Pete, who is looking for the Genie's Lamp. Their fight with him triggers a cave-in. Roxas and Xion manage to escape, only to run into the Magic Carpet and Genie himself. Genie thinks Roxas seems familiar somehow...
You'll begin in the Palace area; both here, and in Agrabah Streets are a few Heartless scattered around. The first real confrontation will happen at Agrabah Gate. A set of four respawning Scarlet Tangos will appear and you will be unable to leave the area. Luckily, the Tangos only respawn once. Between a decent Keyblade and some Blizzard magic from you and Xion, there shouldn't be any issues. When you are ready, head for the Cave of Wonders, also guarded by a few Heartless, and enter.
In the Entrance Hall, you will need to do a little investigating. Clear out the minor enemies, and then head for the northern end and up onto the ledge. There will be a small plaque to the left of the door. Examine it to move things along. You will see three cubes pop up in different locations in the Cave, each bearing a different number of marks. This will trigger the next breakthrough. Head into the Pillar Room and activate the pillars via the switch in the northeast corner. Leap up the pillars, bearing towards the northeast; you are aiming for the high ledge above the pillar switch. Once the enemies around it are gone, hit the switch to activate it. Head for the Abyss. There is a cube in here, but don't hit it yet!. You are looking for the cube with two marks, which is in the high areas of the Entrance Hall, only reachable by jumping off the ledge into the Abyss. So jump on down! When you land, the cube with two marks will be directly in front of you. Get rid of the Heartless, hit the cube, and investigate the nearby crack in the floor. There's also a chest up there you'll probably want to snag. When you're done, back track through the Entrance Hall and Pillar Room to the Abyss and strike the three-mark cube. Voila. The bridge to the other side of the Abyss appears, and another new breakthrough is gained. Open the gate and cross the bridge as fast as you can to enter the new area.
In the Hidden Room, snag the chest and then check out the monkey idol on the south wall of the room to find a dead end. Your mission is done, right? Nope. You have a visitor and boss, in the form of Pete. He is not terribly difficult to take down. Just dodge his fists, deflect his bowling-ball attacks, and jump when he smashes the ground to avoid the shock wave. Once the battle is finished, you will automatically RTC to end the mission.