Set in the Kingdom of Lothric, a bell has rung to signal that the First Flame, responsible for maintaining the Age of Fire, is dying out. As has happened many times before, the coming of the Age of Dark produces the undead: cursed beings that rise after death. The Age of Fire can be prolonged by linking the fire, a ritual in which great lords and heroes sacrifice their souls to rekindle the First Flame. However, Prince Lothric, the chosen linker for this age, abandoned his duty and decided to watch the flame die from afar. The bell is the last hope for the Age of Fire, resurrecting previous Lords of Cinder (heroes who linked the flame in past ages) to attempt to link the fire again; however, all but one Lord shirk their duty. Meanwhile, Sulyvahn, a sorcerer from the Painted World of Ariandel, wrongfully proclaims himself Pontiff and seizes power over Irithyll of the Boreal Valley and the returning Anor Londo cathedral from Dark Souls as a tyrant.

The Ashen One, an Undead who failed to become a Lord of Cinder and thus called an Unkindled, rises and must link the fire by returning Prince Lothric and the defiant Lords of Cinder to their thrones in Firelink Shrine. The Lords include the Abyss Watchers, a legion of warriors sworn by the Old Wolf's Blood which linked their souls into one to protect the land from the Abyss and ultimately locked in an endless battle between each other; Yhorm the Giant, who sacrificed his life for a nation conquered by his ancestor; and Aldrich, who became a Lord of Cinder despite his ravenous appetite for both men and gods. Lothric was raised to link the First Flame but neglected his duties and chose to watch the fire fade instead.

Once the Ashen One succeeds in returning Lothric and the Lords of Cinder to their thrones, they travel to the ruins of the Kiln of the First Flame. There, they encounter the Soul of Cinder, an amalgamation of all the former Lords of Cinder. Upon defeat, the player can attempt to link the fire or access three other optional endings unlocked by the player's in-game decisions. These include summoning the Fire Keeper to extinguish the flame and begin an age of Dark or killing her in a sudden change of heart. A fourth ending consists of the Ashen One taking the flame for their own, becoming the Lord of Hollows.