Grendel, a large bearlike monster, has spent the last twelve years locked in a war against a band of humans. The main action of Grendel takes place in the last year of that war, but the novel skips back in time in order to illuminate the origins of the conflict as well as Grendel’s personal history.

As a young monster, Grendel lives with his mother in a cave on the outskirts of human civilization. A foul, wretched creature who long ago abandoned language, Grendel’s mother is his only kin or companion. One day, the young Grendel discovers a lake full of firesnakes and, swimming through it, reaches the human world on the other side. On one of his early explorations he finds himself caught in a tree. A bull and then a band of humans attack Grendel before his mother rescues him.

Grendel becomes fascinated with the world of men, watching from a safe distance as mankind evolves from a nomadic, tribal culture into a feudal system with roads, governments, and militaries. He is alternately befuddled by their actions and disgusted by their wasteful, brute violence. Grendel watches as Hrothgar of the Danes (also known as the Scyldings, after an illustrious ancestor) develops into the most powerful king in the area.

Eventually, Hrothgar’s power and fortune attract the services of the Shaper, a court bard who sings glorious tales of Danish kings and heroes. Though the Shaper’s songs are only partially based on fact, their imaginative visions of a supremely ordered moral world are incredibly powerful and invigorating. Inspired by the Shaper’s words, Hrothgar builds a magnificent meadhall and names it Hart. Even Grendel, who has witnessed the true, savage history of the Danes, finds the Shaper’s vision extremely seductive and becomes ashamed at his own brute, bestial nature.

Grendel, increasingly upset by his split feelings about the Shaper, visits a dragon in search of some advice. The dragon belittles the Shaper and declares all moral and philosophical systems pointless and irrelevant. Grendel gradually adopts this worldview and becomes enraged at the humans. He begins to raid Hart systematically, initiating the twelve-year war. In his first battle, Grendel handily defeats Unferth, one of Hrothgar’s mightiest thanes (or soldiers), and adds insult to injury by scoffing at Unferth’s romantic ideas of heroism.

Other kings increasingly threaten Hrothgar, who preemptively tries to attack one of them: Hygmod, king of the Helmings. In order to avoid a war, Hygmod offers Hrothgar the hand of his sister, Wealtheow, in marriage. Hrothgar accepts, and Wealtheow becomes the much beloved queen of the Scyldings, bringing a new sense of peace and harmony to the vulgar, masculine world of Hart. The lovely queen briefly enraptures Grendel, and only a nighttime attack and a cold, misogynistic look at her genitals rids him of her spell.

Some years later, Hrothgar’s brother Halga is killed, and Halga’s orphaned son, Hrothulf, comes to live at Hart. Hrothgar and Wealtheow already have two sons of their own, and the presence of so many possible heirs to the Scylding throne makes Wealtheow nervous. Hrothulf, for his part, is disgusted by the split he sees between the laboring class and the aristocracy, and he plans a revolutionary overthrow of the government. Hrothulf’s counselor, a peasant named Red Horse, tries to convince Hrothulf that all governments are inherently evil and that a revolution merely replaces one corrupt system with another.

In the winter of the final year of the war, Grendel watches a Scylding religious ceremony. When all the other priests have left, Grendel meets an old, blind priest and pretends to be the supreme Scylding deity, known as the Destroyer. Grendel asks the old priest, Ork, to say what he knows about the Destroyer, and Ork offers him a complex metaphysical system he has spent years working out. Ork is almost moved to a state of ecstasy by the experience, and a puzzled Grendel withdraws as three younger priests come to chastise Ork for his strange behavior. A fourth priest meets them and is overjoyed at the news of Ork’s vision.

Later the same winter, the Shaper dies. Grendel experiences an increasing feeling of dread, though he cannot place it or puzzle it out. His mother senses it also, and though she tries warning Grendel, she can only produce the gibberish phrase “Warrovish,” which Grendel later deciphers to mean “Beware the fish.” Fifteen strangers arrive from over the sea: they are Geats, and their leader is Beowulf, who has come to rid the Scyldings of Grendel. Grendel knows that the Geats are what he has been waiting for, and he is alternately frightened and excited. The Scyldings are none too pleased at Beowulf’s arrival, and that night at dinner, Unferth taunts Beowulf for famously having lost a swimming contest. Beowulf coldly responds that Unferth has been misled, and calmly declares that Unferth is doomed to hell because he killed his own brothers.

When the Geats and the Scyldings fall asleep, Grendel attacks Hart. Beowulf manages to surprise Grendel and grabs his arm. As they struggle, Grendel slips on a pool of blood, and Beowulf gains the upper hand. Beowulf begins whispering madly in Grendel’s ear. Grendel moves in and out of a series of hallucinations in which he sees Beowulf sprouting an enormous pair of wings. Beowulf smashes Grendel against a wall, cracking his head open and demanding that he “sing of walls.” Beowulf manages to rip Grendel’s arm off at the shoulder, and Grendel runs off into the night. He finds himself at the edge of a cliff, staring down into its dark, murky depths. A host of animals gather around Grendel, seeming to condemn him, and the novel closes as Grendel whispers to them, “Poor Grendel’s had an accident. . . . So may you all.”