For years, the Niflheim Empire and the kingdom of Lucis have been at war. The empire, having dominated most of the world of Eos, covets the power of the last known Crystal, which is held in Lucis' capital city, Insomnia. In order to protect his people from these advancing forces, King Regis Lucis Caelum CXIII used the power of the Crystal to surround Insomnia with a magical wall. Along with this barrier, Regis assembled an elite military task force known as the Kingsglaive. By drawing their power from the king, the Kingsglaive protect Lucis' borders from the onslaught of the empire and other forces that would do them harm. One such member of the Kingsglaive is Nyx, a man nicknamed "The Hero" by his fellow warriors due to his arrogance and desire to save everyone. However, his pride gets the better of him, causing him to disobey his captain's orders, resulting in a demotion. Now, Nyx spends his days guarding the city gates, but things begin to change once word gets out that Regis plans to sign a peace treaty with their sworn enemies.

Two years ago, the world was changed forever. The young Cloud Strife and his band of friends may have defeated Sephiroth and thwarted his plan to crash a giant meteor into the Earth, but this victory was not without great cost. The highly populated city of Midgar was nearly ripped apart in the conflict. Fortunately, many of the city's citizens were able to evacuate to safety, and in the years afterward have formed a new home called Edge. Final Fantasy VII: Advent Children revolves around Cloud and Tifa as they try to make new lives for themselves, as well as for those around them, in this new city. Together they run a courier service, and tend to their mutual friend Barret’s adopted daughter Marlene and a young orphan by the name of Denzel. Denzel, Cloud, and scores of children are suffering from a mysterious new illness called "Geostigma." The children of the city have one other threat looming over their heads—a trio of powerful men are kidnapping infected kids for unknown reasons. Cloud is determined to save these young ones, not only from Geostigma, but from the kidnappers as well. He has no idea, however, that these men share a link to his old enemy, Sephiroth, and Cloud’s quest to vanquish them will bring him back into conflict with the demons of his past. If there is any hope in conquering these threats, it lays within the bonds of friendship between Cloud and his allies who saved the world once, and now must do so again.