You’ll talk to Barret and then find out that Meteor has been summoned, WEAPON’s are attacking and that the Northern Cave is protected by a barrier. When you can move talk to Barret, and Rufus and Heidegger will turn up. As Barret, follow Tifa and the guards. Save at the Save Point then follow them down the stairs. Go left and down the stairs. Just keep following them, until you come to a room with some chairs. Tifa will get strapped into the gas chamber. WEAPON will then attack Junon. Scarlet will get knocked out by Cait Sith and two guards will attack Cait Sith and Barret. Just attack with physical attacks. After the battle, go to the door in the south east. Press O a few times. You’ll then see Heidegger and Rufus arm the big cannon to fire against WEAPON.
After it fires and WEAPON still attacks Junon, the scene will switch back to Tifa. Gas will start pumping into the chamber. As Barret go talk to Cait Sith. Go north, out the door, then right. There can be some tough random battles around here, so you may want to equip some materia. You’ll see Yuffie posing as a reporter, talk to her to get her to join you. Keep going left. Go north and examine the yellow box to go up a level. Go north east towards the ship in the background. You’ll now switch to Tifa. To free her, press the following buttons: X, X, Triangle, X and Triangle, Triangle and Circle, Circle. This will free her and then go around to the back of the seat and press O to shut off the gas. Go over to the door and WEAPON will cut and opening in the ceiling before being shot again with the cannon. Make your way down the side and then go towards the top left, using the Select button might help. Keep going this way to the end of the cannon. Scarlet will appear and start slapping you! Slap her back with O. If you hit her enough, she’ll fall to the ground; if you get hit too much, you fall to the ground. No matter what happens, Barret will appear in the Highwind and save Tifa.
Go into the door to the left to enter the Highwind. Go down the stairs and go to the right along the walkway. Keep going right to get to the cockpit. Talk to everyone here, ending in Red XIII. Then talk to Cid and the pilot next to him. Go back to the room called Operation. Talk to the man here, and make a new party. You can also restore HP and MP, and save. Go back and talk to the pilot and choose the top option to fly the Highwind.
You’ll notice the big pink Meteor in the sky and you can ram the barrier around the Northern Crater (it won’t get you anywhere). Lots of places sell new items, such as Junon Town, Fort Condor, Costa De Sol and Rocket Town. In Rocket Town, in the weapon/accessory shop there is a chest with a Fourth Bracelet in. When you’re done, fly to the south-eastern continent. There is an town surrounded by a forest. Land the Highwind on the grass and enter the town.