Serah, the younger sister of Lightning - the protagonist of FFXIII, wakes from a disturbing dream of Lightning in some unknown world fighting and leading an army of monsters and eidolons. She wakes into a world where only she remembers her sister surviving the catastrophic events that concluded Final Fantasy XIII. It has been three years since that day. Serah has never given up on her memories of her reunion with her sister. Only Snow believed her and he has disappeared on a journey with a promise to find Lightning. Despite staying with the members of NORA, she feels alone until a momentous day when a comet crashes into her home on Pulse in New Bohdum. Along with the comet and a hoarde of monsters arrives a strange young man named Noel who claims to have been sent by Lightning. He asks her to go with him with the promise that he will take her to Lightning. Together they start on a journey that will take them through paths of time and paradoxes that twist both the future and the past.

Noel is a mysterious young man who falls into time from his home many ages from Cocoon's fall where he was the last survivor of humanity. He appears in a place called Valhalla where it seems Lightning has been expecting him. She sends him on a desperate mission to find her sister, Serah, and bring her to Valhalla. Noel travels through time to find Serah. Serah joins him on the quest to find Valhalla. As the two travel through time and resolve paradoxes Noel struggles as he begins to question his own memories and is haunted by memories of a young woman named Yuel.

Mog is a curious creature sent to Serah by Lightning with Noel when he arrives in New Bohdum. He travels with Noel and Serah. He has the convenient ability to transform into both a bow and a blade that Serah uses in battle. He has other unique abilities that help Serah on her journey and despite being a little scatterbrained, appears to know quite a bit about the paradoxes and time distortions that Serah and Noel discover.

Lightning was the lead heroine of Final Fantasy XIII. Sometime after her happy reunion with Serah she disappears into time. All memories of Lightning surviving the rescue of Cocoon are wiped from the memories of those that knew her. Only Serah remembers and even then she has doubts about her own memories. But Lightning survives in a place called Valhalla, a world that seems to exist outside of time. She faces a great threat from a man named Caius and sends the mysterious Noel on a desperate mission to bring Serah to Valhalla.