After the mysterious disappearance of one of its participants, the battle meant to name a successor to God is brutally interrupted, leaving behind a trail of destruction and shaking the world. Twelve years later, while the former contestants in the divine competition struggle to fit in society after they were turned into semi-permanents—beings unable to age—Lall refuses to let go of her brother, who has been forgotten by everyone. Determined to resurrect her brother—the only one she sees fit to sit on the heavenly throne—Lall steals her his cellphone from a cult run by a past competitor, which accidentally restarts the competition. Helped by her classmate Eko Sasaki, Lall will show no mercy to prevail against her enemies in her efforts to finally see her brother again.

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.

Reeling from her recent breakup, Momo Ayase, a popular high schooler, shows kindness to her socially awkward schoolmate, Ken Takakura, by standing up to his bullies. Ken misunderstands her intentions, believing he has made a new friend who shares his obsession with aliens and UFOs. However, Momo's own eccentric occult beliefs lie in the supernatural realm; she thinks aliens do not exist. A rivalry quickly brews as each becomes determined to prove the other wrong. Despite their initial clash over their opposing beliefs, Momo and Ken form an unexpected but intimate friendship, a bond forged in a series of supernatural battles and bizarre encounters with urban legends and paranormal entities. As both develop unique superhuman abilities, they learn to supplement each other's weaknesses, leading them to wonder if their newfound partnership may be about more than just survival.