Set in the 60 year interim between the events of The Hobbit and The Lord of the Rings, the story follows Talion, the captain in command of the garrison of Gondorian Rangers charged with watching over the Black Gate of Mordor. As well as the men under his command, Talion's wife Ioreth and their teenage son Dirhael, who seeks to follow in his father's footsteps, reside there. On the night Sauron returns to Mordor, having been driven out of Dol Guldur, the garrison is attacked by Orcs under the command of three Black Númenórean captains: the Hammer of Sauron, the Tower of Sauron, and their leader, the Black Hand of Sauron. Talion, Ioreth and Dirhael are the only survivors of the attack; they are taken prisoner and ritually sacrificed by the Black Hand in an attempt to summon into himself the wraith of an unknown Elf lord. Instead however, the elven wraith bonds with Talion, preventing him from dying with his family. The pair agree to work together to find answers regarding the elf lord's past, avenge the murders of Talion's family and kill the Black Hand, whose death will break the spell binding them together, allowing them both to die.
Over the course of their travels, the pair encounter Gollum multiple times, who is wandering Mordor in search of his "Precious" (The One Ring). Able to see and speak with the wraith due to his prior contact with the One Ring, whom he refers to as the "Bright Master", and hoping they might lead him to the One Ring, Gollum leads the elf and Talion to relics of his past, each of which restore parts of his lost memories, eventually allowing the elf to remember his true identity: Celebrimbor, the greatest jewelsmith of the Second Age. Celebrimbor gradually recalls how Sauron, disguised as Annatar, the Lord of Gifts, deceived him into forging the Rings of Power. Celebrimbor ultimately assisted Sauron in forging the One Ring, but was able to steal it from him. Celebrimbor proclaimed himself the Bright Lord of Mordor and raised an army of Orcs against Sauron with the intention of both rescuing his family (Sauron having taken his wife and daughter hostage to force Celebrimbor to aid him) and deposing Sauron.
In his search for the Black Hand, Talion allies himself with Hirgon, a ranger deserter who leads a community of Gondorian outcasts choosing to settle in Mordor, and Ratbag the Coward, an Orc who offers to bring Talion closer to the Black Hand in exchange for his assistance in climbing Mordor's military hierarchy. Talion helps Ratbag ascend the ranks to the level of warchief by killing each of his immediate superiors, allowing Ratbag to become a Warchief himself. Talion then goes after the other Warchiefs in Gorgoroth, weakening Sauron's forces in that area. Soon enough, Hirgon has a plan to draw out the Black Captains. Using acquired grog and blasting powder, Talion helps the Outcasts destroy a statue dedicated to Sauron. Instead of the Black Hand answering the "message", the Hammer of Sauron is sent, punishing Ratbag for not answering his duties to kill the ranger. Talion and The Hammer then face off, with packs of Uruk joining the fight, but after a savage battle, Talion manages to kill The Hammer.
Talion is then sought out by the warrior Lithariel, the daughter of Queen Marwen, who claims to be able to assist Celebrimbor with his mission. Marwen is the ruler of Núrn, a kingdom of sea raiders located around an inland sea in the south of Mordor. She uses her powers to guide Talion and Celebrimbor to another of Celebrimbor's relics. Talion also gets help from a Dwarf hunter named Torvin, teaching Talion the ways of the hunt to dominate Caragors and Graugs (beasts native to Mordor), eventually fighting a massive Graug from Torvin's past. Marwen advises them to use Celebrimbor's powers to take control of an army of orcs and other beasts, using them to lead an assault against Sauron. Talion eventually realizes that the wizard Saruman is possessing Marwen, and assists Lithariel in freeing her from his control. Talion, however, still carries out Saruman's plan, leading an army of mind-controlled orcs in an assault against the Black Hand's stronghold at Ered Glamhoth. However, rather than the Black Hand, Talion finds the Tower of Sauron waiting for him. The Tower mockingly taunts Talion that Celebrimbor chose him as a host and can free him at any time and offers the elf a place at Sauron's side. Refusing the offer, the two battle and Talion emerges victorious after viciously stabbing the Tower to death with his son's sword. Afterwards, Celebrimbor confesses that the Tower told the truth and promises to let Talion join his family in death if he wishes, once the Black Hand is slain.
As Mount Doom erupts into life for the first time in generations, Talion travels to the Black Gate for a final confrontation with the Black Hand. The Black Hand quickly incapacitates him with a spell that also restores the last of Celebrimbor's memories revealing that in his final confrontation with Sauron, the One Ring ultimately betrayed Celebrimbor and returned to its true master, who swiftly defeated him. A victorious Sauron then punished Celebrimbor by forcing him to watch the murders of his wife and daughter before finally killing Celebrimbor with his own smithing hammer. The Black Hand then kills himself as part of a ritual that forces Celebrimbor to depart from Talion and merge with himself. This allows Sauron to possess the Black Hand's body and incarnate in physical form. However, Celebrimbor is able to briefly paralyze Sauron from within, allowing Talion to destroy Sauron's physical form. With the Black Hand dead, Celebrimbor wishes to depart for Valinor. Talion instead convinces him to stay and attempt to overthrow Sauron. Gazing at Mount Doom, Talion declares his intention to forge a new Ring of Power.