Four decades after the Oblivion Crisis, an unknown mass appeared on the coast of Black Marsh during a powerful storm. Meanwhile, a Dunmer assassin named Sul awakens from a nightmare in which he witnessed this appearance.
Annaïg Hoïnart, a seventeen-year-old Breton girl, and her Argonian friend Mere-Glim, drinking her father's wine after a night of adventure, talked about the spreading rumors of Umbriel, the flying city. Though her father Taig tried to have her taken to Leyawiin, she used her magical bird Coo to bring Glim to her rescue. During their flight from her captors, they witnessed Umbriel moving toward the city. Glim was unable to stop himself from moving toward the mass of land; to avoid being dragged with him to the chaos below, Annaïg took a flying potion, poured one into his mouth, and guided them the only way she could — toward Umbriel. They witnessed the slaughter below and when they realized they had been seen, she sent Coo to find Prince Attrebus Mede. Annaïg and Glim were taken captive by the residents of Umbriel, and Annaïg was made to cook while Glim was trained as a skraw, to work the Sump.
Rising from the bed he shared with Radhasa, a Redguard woman whom he had just signed on to his guard, Attrebus found Coo on his balcony. He spoke with Annaïg, who told him of the coming danger. When his father, Emperor Titus Mede, brushed off the situation since Black Marsh was no longer a concern to him, the prince secretly gathered his guard. Near the beginning of his journey, however, his men were ambushed and Attrebus was taken captive by Radhasa and her men.
Colin, a young Inspector for the Penitus Oculatus, inspected the area where Attrebus was attacked. He discovered a decapitated and charred body with Attrebus' Imperial signet ring, but thought it was too convenient to be authentic. Meanwhile, Attrebus awoke tied up and riding a horse. When they came to a stop, a small skirmish that Radhasa was winning against Attrebus' escape attempts was interrupted by Sul, who charged the group and easily dispatched the small group of traitorous allies.
Colin revealed to the Emperor that he thought the body recovered was not that of the Prince before heading to a tavern in the Market District, where he met an old friend Nial Sextius. While discussing with him the disappearance of the Prince, he discovered that Gulan, Attrebus' adviser and assistant, would have reported Attrebus' plans to the Prime Minister's office, not directly to the Emperor.
Back on Umbriel, the kitchen Annaïg worked for was attacked by the Toel kitchen. She and Slyr were taken captive and trained to work for his kitchen. During the attack, Annaïg lost the locket that connected her to Coo, but accidentally acquired Qijne's invisible, possibly biological, blade, which wrapped itself around her forearm and extended as she wished.
In an outdoor area of Umbriel that Glim discovered called the Fringe Gyre, he found trees that appeared to be related to Hist, since he could hear them murmuring. Here he also met a girl, Fhena, who was able to confirm for him that Annaïg was alive and was the subject of a large battle between the kitchens. The same evening, he talked with Wert and other skraws. They discussed ways of "negotiating" better treatment, and Glim asked them to make maps of Umbriel.
During their journey, Sul and Attrebus met a group of Khajiit who joined them. Sul explained to Attrebus that he intended to short-cut through Oblivion to beat Umbriel to Vvardenfell. He explained that Umbriel was undoubtedly a product of Vuhon, the man who had engineered the ingenium, the device that had kept the Ministry of Truth in place in Vivec's absence. He killed Sul's love, Ilzheven, and caused the Ministry of Truth to crash into Vvardenfell, in turn causing Red Mountain to erupt and destroy the entire island. Sul and Vuhon both were thrown into Oblivion, and a being named Umbra took that opportunity of the barrier between worlds opening to throw a sword, also called Umbra, into Tamriel, preventing Clavicus Vile from imprisoning him in it again.
They traveled to Water's Edge for supplies, and while there Attrebus tried to garner support from an old friend, Captain Florius Larsus. Florius agreed to join Attrebus, but while waiting for him in a tavern Attrebus was attacked by a group of people hired by the same that hired Radhasa and her allies to kill him. He found that one of the men that attacked him worked for Florius and ran back to find him sitting at a table, a knife wound in the base of his skull.
Sul opened a portal to Oblivion and he, Treb, and the Khajiit entered the planes of Oblivion, traveling through different planes, noticing the differences through the pain of the transitions. The path was interrupted in Hircine's plane, and Sul guided the party through Oblivion as Hircine and his drivers hunted them. One of the Khajiit was killed during the chase, and those remaining stayed behind at the end of the course to delay Hircine from reaching Attrebus and Sul.
Attrebus and Sul exited Oblivion directly into the ruined Vivec City. There, Sul explained that Azura told him he'd find Umbra here, and that it must be what Vuhon was after. He found that the place where Umbra should have been was gone, and spoke with the spirit of Ilzheven. He questioned her about Umbra, and she told him that Dunmer found it and took it north, toward the Sea of Ghosts.
As Sul wandered the ruined city, Attrebus kept an eye out for Umbriel and opened Coo, finding Annaïg looking back at him. She told him that she couldn't wait for him, that she and Glim would use her flying potions and leave Umbriel. When she closed her locket, a few moments later Attrebus saw the bottom of Umbriel poking out from the bottom of the clouds, heading toward them and very close.
Annaïg, working in Toel's kitchen on Umbriel, received a bucket of ingredients from a skraw that contained her locket, and inside it a message from Glim. She responded to Glim's note and they met the following midnight, at the dock. They consumed flying potions Annaïg had made, and begin to fly away from Umbriel, only to find that they had become like the other residents of the city; if they traveled too far from Umbriel, they began to lose their substance.
Though Colin had been pulled from the Attrebus investigation, he continued investigating. The last person Gulan had spoken to, Letine Arese, an assistant to the Prime Minister, made her way deep into the Market District and entered a building, and Colin climbed a nearby building in order to reach a third-story window. He entered a small empty bedroom and made his way downstairs until he heard the voices of Arese and a male. She seemed concerned that the "job" was not accomplished as it should have been. Colin learned that a courier brought news to the Emperor that the Prince lived. As their conversation escalated, among the screams of men it seemed that Arese changed form into a sort of beast. It moved past him, up the stairs, then a few moments later came back down and exited through the door. The building was burning, and Colin escaped through the window. Unable to search the house, he wondered why Arese wanted the Prince dead, and why she didn't accomplish the task herself.
Sul was drawn back to Attrebus by his screams, to find Umbriel nearly upon them. They attempted to move back to their entry position, but were transported to Umbriel, suspended in a type of spiderweb-like net in front of Vuhon. As they spoke, Sul accused Vuhon of murdering his love, Ilzheven, and destroying Vvardenfell; Vuhon contested that it was actually Sul that did that. It was revealed that Ilzheven had a very special type of soul, and Vuhon was using her to power the ingenium, to hold the Ministry of Truth aloft over Vivec City. When Sul freed her, it sent the Ministry of Truth crashing into Vivec City, killing Ilzheven and throwing Sul and Vuhon into Oblivion. Vuhon also revealed that he had an ally inside the Imperial Palace, the person that was trying to have Attrebus killed. He said he'd need the Imperial City, specifically the White-Gold Tower, and offered Attrebus the chance to convince his father to back down. Through his close relationship with Oblivion, Sul shattered Vuhon's glass world by summoning a monstrous daedra. While Vuhon was distracted, Treb tried to attack from behind, but was discovered by Vuhon and trapped once more. At the last moment, Sul sprang to the prince's side and said "Not now," before laying a hand on his shoulder and pulling them both into another part of Oblivion.
The book ends with Glim and Annaïg back on Umbriel, defeated. They spent a few hours together before saying their goodbyes, and Annaïg headed back to the kitchen. She promised Glim that, while all they could do was continue to move forward, eventually they would be free. She then resolved to become stronger and more ambitious in order to survive.