Final Fantasy Advent Children
The following lists all events of Advent Children in stages. This will include spoilers.
Geostigma
The disease known as Geostigma is spreading across the world. Cloud Strife and Tifa Lockhart now work as couriers delivering goods all over the land, and all the other members of the FF7 cast have scattered across the land, resuming their lives before the Sephiroth incident. It is also discovered that Cloud also has the Geostigma disease, as well as Denzel, a close friend of Marlene, Barrett's daughter.
There is little known about Geostigma, save that it appears physically as a strange skin disease that would probably spread across the body in time. Nothing in the movie mentions it being fatal, but there is a good chance that it does, since many of those afflicted eventually become bedridden. The Geostigma disease apparently, has also appeared shortly after the prevention of Meteor, so it can be assumed that Geostigma is the result of an imbalance found in the Lifestream.
A majority of those infected with the Geostigma, also happen to be children.
the Turks
Cloud is summoned by a hooded man in a wheelchair, seemingly the head of what remained of the Shinra employees, since the Turks Reno and Rude follow his orders. At this point in time, it can only be speculated that the hooded man in a wheelchair is President Rufus, thought to have died during the attack on Shinra company during the events of Final Fantasy Seven. The man also apears to be suffering from Geostigma. Cloud refuses to work for him.
It was believed that Reno and Rude are the only surviving Turks. Earlier on, Reno was seen on a helicopter flying towards the Northern Crater, where other fellow Turks Tseng and Elena seem to be exploring, and obviously searching for something. A few moments after the helicopter disappears into the crater's mist, Elena and Tseng are attacked; Elena screams at Reno to go and leave them. The helicopter reappears, flying away from the crater, but without Elena and Tseng in it, presumably dead.
the Sephiroth clones
Later, Cloud is ambushed by three silver-haired men who bears a striking resemblance to Sephiroth: Kadaj, Loz, and Yazoo. It is unknown how these three men came to be, but it is obvious that they have something to do with Geostigma, and that they are after their 'mother' Jenova's head, to created another 'reunion' - the same as what Sephiroth had planned in the events of FF7. It is evident that Sephiroth's spirit resides in all of them, possibly in Kadaj most of all. They seem to have a partnership with the hooded man from Shinra, and it is soon discovered that the Turks were in the Northern Crater to hunt for Jenova's head. Kadaj demands Jenova's head, producing Tseng and Elena's bloodstained IDs, revealing that it was they who attacked the Turks in the Northern Crater. The hooded man tells the clones that they lost Jenova's head while escaping, but implied they would aid Kadaj in finding it again.
The three men now begin to round up all the children infected with Geostigma, promising to take them to a place to be cured. Loz encounters Tifa Lockhart and Marlene in the abandoned church (where Aeris once sold flowers in, and where Cloud is currently taking residence in). Tifa fights Loz, but eventually loses, and Marlene is kidnapped despite not being infected with the Geostigma. Loz also finds Cloud's stash of materia, which he takes. The three go to the City of the Ancients - the same place in FF7 where Aeris went to pray for Holy materia (and was killed by Sephiroth), and Kadaj managed to control the children with the water from the Ancient's pond.
Vincent Valentine
Cloud finds the unconscious Tifa, then collapses himself, weakened by the Geostigma, where he encounters Aeris in what seems like a dream. Waking up, he discovers that they are saved by Reno and Rude, who in turn tell them that the three men have gone to the City of the Ancients. Cloud starts after them, and still weakened, is almost defeated by the three clones, until Vincent intervenes in a flurry of red cloth, and whisks Cloud away to safety.
Vincent tells Cloud later on that he often saw the clones since he staked out the area, and that it was he who found Elena and Tseng, both badly tortured, and saved them. Cloud initially wants to go alone, but Vincent convinces Cloud to go back and call on the others on the team for help.
Bahamut
Kadaj believes that Jenova's head had been hidden inside a large statue in the center of Midgard, and uses his control over the children to make a barrier, summoning monsters to clear the crowd of terrified people who have gathered. Reno and Rude fight the three, but prove to be the weaker team, until the rest of the old FF7 team reappears: Barrett Walace, Red XIII and Cait Sith (Reeves is still using Cait Sith as his walking megaphone, though where Reeves really is physically is unknown), Yuffie Kisaragi, Cid Highwind (who saves Tifa Lockhart and Denzel), Vincent Valentine, then lastly Cloud.
Kadaj, using the materia Loz took from Cloud, decides to summon Bahamut, and it takes the combined might of the whole team (together with Cloud's final attack, and a big boost courtesy of Aeris' spirit, still watching over Cloud and the team) to beat the summoned creature.
Rufus Shinra
The hooded man and Kadaj are in an abandoned building watching the fight, when the hooded man stands up from his wheelchair, revealing himself to be a heavily bandaged Rufus himself - also holding Jenova's Head, which he had been hiding all along. It was a trap set by Rufus all along; he attacks Kadaj, aided by Elena and Tseng, and throws Jenova off the building, but Kadaj manages to rescue it and flees.
Sephiroth
Cloud chases after Kadaj, with the team watching from Cid's airship, Shera, but Kadaj breaks the seal on Jenova, and transforms mid-fight into Sephiroth. Sephiroth now reveals his plan to Cloud - he wants to use Geostigma to fuel the world and transform it into his spaceship of sorts, so he could do the very thing Jenova had once did - travel to other planets to conquer.
In the course of the fight, Sephiroth succeeds in wounding Cloud, taunting and asking him how he'd gotten so strong. This is something of deja vu; every time Sephiroth and Cloud engages in battle, and everytime Sephiroth manages to wound Cloud, he becomes overconfident, now certain of his own victory. Cloud responds, telling Sephiroth that he was strong because he had others to support him where Sephiroth had none. He then executes a very highly evolved version of the Omnislash (very different from the Omnislash in the game - this may be an indication that Cloud has gotten stronger in the past two years, and that his Omnislash only reflected that additional strength). Sephiroth's wing springs out, and he tells Cloud "I won't be a memory." shortly before the wing envelops him, transforming him back to Kadaj. Kadaj tries to fight, fails, and hears a voice he believes is his mother, telling him to return to the Lifestream. He slowly begins to evaporate.
Aeris Gainsborough
In the meantime, a light rain begins to fall, and the people stricken with Geostigma, including the children kidnapped, Cloud, and Rufus, are cured when bathed in the rain. Tifa, Marlene, and Cloud are quick to realize that this was Aeris' spirit's doing. (Marlene in fact, is assumed to have actually seen Aeris during this time, possibly because she says Aeris was her favorite person; she calls out "Oneesan!")A rapidly evaporating Loz and Yazoo ambushes and attacks Cloud, resulting in a huge explosion along the area. While Cloud drifts, unconscious, he hears two voices talking to him - Aeris' and Zack's spirit. Upon being called "Mother?" by Cloud, Aeris wonders why she was always being called that, to which Zack replies that it just means that she's loved. They bring Cloud back to the abandoned church, onto a pond inside that had sprung up recently. Coming to, Cloud realizes that this is Aeris' healing pond; his wounds are healed, and all the children with Geostigma are healed when bathed in the water.
Looking around, surrounded by his friends, Cloud catches sight of Aeris' spirit walking away from the group, towards the main doors, where Zack's spirit was waiting. She turns, smiles at Cloud and asks "It's alright, isn't it?" As both Aeris and Zack disappear into the light, Cloud realizes that he is not alone.