story FFXIII-2

The game opens in 3 AF, as the Pulse town Serah lives in is attacked by monsters. A stranger named Noel appears to help fight the monsters and claims to be a time traveler from 700 AF. He arrived in her time via Valhalla, where he claims to have met Lightning as she guarded the throne of the weakened Etro. As part of Lightning's transfer to Valhalla, she was erased from the fall of Cocoon onwards (making everyone except Serah forget her being with them on Gran Pulse), and paradoxes have erupted throughout time, warping the timeline. Serah joins Noel in a journey to resolve these paradoxes by removing items and monsters, which are out of their original time and correct the timeline; she in hopes of finding her sister and he in hopes of changing the bleak future he comes from. While journeying to 5 AF to resolve a paradox on Cocoon, they meet and help Alyssa, an Academy member and survivor of the Purge, a massacre by the Sanctum at the beginning of Final Fantasy XIII.
Traveling to the ruined city of Paddra in 10 AF, they find Hope, who has become the leader of the Academy, with Alyssa as his assistant. They also find a recording of prophecies made by one of the seeresses of Paddra, who are believed to have died out centuries prior: one fragmented prophecy shows Lightning in Valhalla.Paddra is shrouded by an eclipse, which Noel says is not supposed to happen for several centuries. While Serah and Noel resolve the paradox causing the eclipse, they encounter Caius Ballad, a man Noel knows from 700 AF who opposes their mission, and Yeul, who looks identical to a girl of the same name Noel knew in the future. After resolving the paradox, an alternative timeline appears in which there was never an eclipse. The repaired prophecy shows Caius in Valhalla fighting Lightning and the pillar supporting Cocoon collapsing. Noel claims that this takes place around 400 AF, devastating the human population as well as the world, creating a future where he is the last human in existence. Serah and Noel move on, while Hope and Alyssa work on finding a way to prevent the pillar's collapse.
Moving on to 300 AF Cocoon, the pair find Snow fighting a giant paradox-fueled monster that is dissolving the crystal pillar. After resolving the paradox, which delays the fall of Cocoon until 500 AF, Snow disappears again as an anomaly from another time. The pair then go to the city of Academia on Gran Pulse in 400 AF where they are attacked by the city's AI, which claims they were killed in 200 AF. Traveling to 200 AF, they discover a paradox whereby a man-made fal'Cie meant to re-levitate Cocoon was able to turn on its creators using the AI that designed it and take over the government. Resolving the paradox, Noel and Serah travel to an alternative 400 AF Academia. In this world, the pair find Hope and Alyssa again, who had put themselves in stasis. They explain their new plan to mechanically float a new Cocoon to hold humanity, which Serah and Noel help with before heading to 500 AF Academia. They are betrayed by Alyssa (revealed to be a living paradox doomed to disappear in the corrected timeline) and trapped by Caius in dream-worlds. Before being trapped, Serah meets the spirit of Yeul, who explains that she is the seeress of Paddra, continually reincarnated throughout history, while Caius is her immortal guardian, gifted with Etro's own heart
Yeul explains that every time the timeline is changed, the resulting shock kills her; Caius has been driven mad by watching her die repeatedly and seeks to end the process by unleashing the chaos trapped in Valhalla to destroy all time. Serah, it is revealed, has the same power, and risks death every time she changes the future.Resolving to go on, Serah breaks free of her dream-world and frees Noel from his, in which he is the last living human after his Yeul dies and Caius leaves for Valhalla. After briefly encountering Lightning, the pair fight Caius, first in 500 AF Academia and then in Valhalla. As they defeat him, he claims to have killed Lightning, then impales himself through the heart on Noel's blade, killing the weakened Etro.When the pair return to Academia in 500 AF, where Vanille and Fang have been rescued from the collapsing pillar and the new Cocoon, named "Bhunivelze", has risen, Serah dies in front of Noel and Hope from the shock of the future changing.Just then, a black cloud erupts from the sky as the chaos that Etro was keeping trapped breaks free. Lightning is then shown in crystal stasis on the throne of Etro's temple. If the player completes all optional parts of the game, they are shown an additional scene in which Caius is on the throne, declaring that the goddess is gone for good, and that Yeul and he can begin a new life freed from their 'curses'.