Find: FullCure Materia, 1 Elixir, & 1 Magic Source.

Fly to Cosmo Canyon and enter. Go to the Item Shop and run around to the right. Go back here for the FullCure Materia, and an Elixir and Magic Source. Now go up to Bugenhagen's observatory. Enter the door on the right that leads to the Planetarium. Talk to Bugenhagen. The whole party comes in. After some talking, you put the Huge Materia up in the observatory. After looking at it, come down and you end up in the Highwind.

Go to the Forgotten City on the Northern Continent. Enter, and take the left path. Go around inside the large cave and get to the large blue crystal. Talk to Bugenhagen. You learn that there is a key. If you keep coming back to Bugenhagen, these will be the clues he gives:

• A (place where even sunlight can't reach) where the (key) is...
• (even sunlight can't reach)... Hmm.. if we're above ground, and sunlight reaches everywhere... Then it's probably not above ground...
• A place where sunlight doesn't reach... Right, it must be underwater!

Head out to the Highwind, then go to your submarine. Get in and head north from the dock. Follow the dark blue water up to where the large white peninsula sticks out of the Northern Continent.

Dive, and follow the tunnel east to a large object underwater. Go into it with the sub, and you find that it is the Key to Ancients. Get it, and return to Bugenhagen. Talk to him to see some FMV of Aeris and the White Materia. Leave, then Cait Sith calls on the PHS. You find out that the Sister Ray has been moved to Midgar in a cool FMV sequence.

Get back in the Highwind. A Weapon rises up from the sea and heads towards Midgar. Go to Midgar and land the Highwind. Stand on the coast and fight the Diamond Weapon as he approaches.

Use the strongest attacks that you have, and Cure/All often. If you kill him before he finishes his Countdown attack, you win. If not, the Sister Ray finishes him off.

Actually, either way he gets killed by the Sister Ray. But beating him gives you so much experience and ability points, it's dumb not to kill him. The huge shot from the Sister Ray goes right through the Diamond Weapon, and into the shield on the Northern Continent. It destroys the shield. The energy that Diamond Weapon shot before he got destroyed hits Midgar, killing Rufus.

You can now go fight the Ultimate Weapon, or head to the crater on the Northern Continent. Fly there, and you automatically see Hojo take over the Sister Ray, therefore you head to Midgar to stop Hojo.