Final Fantasy 7's storyline is an extremely complex storyline when it comes down to it. Each individual character has a monstrous backstory, and the game's story itself allows for an enormous amount of debate and intrigue. As each new character is introduced, their story intertwines with the greater whole, making the most innovative storyline yet. Warning: Here be spoilers.
Final Fantasy 7's story begins many years before the game does. Shinra, the world's biggest electric company, has come up with plans to help expand their enterprise. The recent discovery of Mako as an energy source has led the Shinra company to create 7 monstrous reactors to surround their giant mechanical city, Midgar. These reactors run day after day, sucking the Mako, or life energy, out of the earth. The Shinra executives want nothing more than to expand their already gigantic company, so they foray into distant lands in search of more Mako, not realising that the earth is dying every second.
A rebel group named AVALANCHE quickly finds out about the threat to the earth and bands together to put a stop to it. As Final Fantasy 7 begins, several members of AVALANCHE and one ex-shinra mercenary attempt to blow up one of the reactors that power Midgar. After the reactor is succesfully destroyed, Cloud and the others return to Tifa's 7th Heaven, in the Midgar Slums below the gigantic plates that make up the city of Midgar. There they meet Tifa, a long time friend of Cloud, who wishes to help with the next operation, again to destroy a reactor powering Midgar.
The rebel group heads out again to destroy the reactor and successfully does so. However, on their way out they come across none other than President Shinra himself, who has learned of this alliance and wishes to put a stop to it. A quick fight and an explosion later has Cloud fall down a gigantic gap, back into the slums and into a church, onto one pretty flower girl's flowers. Said flower girl, Aeris, quickly assists Cloud and takes him to her house to get better and rest. The next morning the both of them head back to Don Corneo's Mansion, where they had previously learned that Tifa was being held captive.
At Don Corneo's Mansion, Tifa, Cloud and Aeris learn as much as they can before being plummeted into the sewers. There, they make their way back to Tifa's 7th Heaven only to find that Shinra was planning on dropping the plate! Quickly, Cloud and Tifa run up the central pillar to attempt to save everyone, but fail. Tifa, Barret and Cloud barely escape with their lives, with Aeris the flower girl captive in the hands of the Shinra.
Cloud and the others quickly head towards the Shinra building itself, where they rescue Aeris and meet with a new character, Red XIII. However, one horrible event happens and the party finds themselves in the hands of one of the most powerful people in the world: Sephiroth. Sephiroth has slaughtered the entire building, killed the president, taken the head of Jenova and still escaped. Enter Rufus Shinra, previous Vice-President and now President, to take command of everything. After a rough battle, Cloud, Tifa, Barret and the others escape not only with their lives, but with a new adventure in hand.