In the opening scene, Grendel briefly fights with a ram when frustrated with its stupidity. He then mockingly asks the sky why animals lack sense and dignity; the sky does not reply, adding to his frustration.[a] Grendel then passes through his cave and encounters his mute mother before venturing out into the night where he attacks Hrothgar's mead hall, called "Hart" in Grendel. Later, Grendel reminisces about his early experiences in life, beginning with his childhood days of exploring the caves inhabited by him, his mother and other creatures with whom he is unable to speak. He goes on to recount an experience from his childhood, in which he found himself painfully wedged in a tree and threatened by humans, only to be saved by his mother.
During Hrothgar's rise to power, a blind poet appears at the doors of Hart, whom Grendel calls "the Shaper".[b] He orates a myth in which the ancient warrior Scyld Shefing seduces of Grendel. Though the story is fabricated, Grendel, listening from afar, is moved by it, and is frustrated by his emotional engagement with the Shaper's untrue stories.
When Grendel returns to his cave, he fails to communicate with his mother, leaving him feeling isolated. He floats down a subterranean river to a dragon's cave. The omniscient dragon discusses with Grendel the oratory power of the Shaper. The dragon and Grendel have a philosophical disagreement about the dragon's existential nihilism, and Grendel exits the cave angry and confused.
While listening to the Shaper, Grendel is spotted by sentries from Hart. In the fight that ensues, Grendel discovers that the dragon enchanted him, leaving him impervious to weapons. Realizing his power, he begins attacking Hart. Grendel is challenged by a thane named Unferth, whom Grendel simply mocks and refuses to fight. Grendel awakens a few days later to realize that Unferth has followed him to his cave in an act of heroic desperation. Grendel continues to mock Unferth until the thane faints from exhaustion, then takes him back to Hart to live out his days in frustrated mediocrity, thereby depriving him of the heroic death which he desired.
In the second year of his conflict with the humans of Hart, Grendel notes that his raids have destroyed the esteem of Hrothgar, allowing a rival noble named Hygmod to gain power. Fearing deposition, Hrothgar assembles an army to attack Hygmod and his people, the Helmings. Instead of a fight, Hygmod offers his sister Wealtheow to Hrothgar as a wife after a series of negotiations. The beauty of Wealtheow moves Grendel as the Shaper had once before, impeding the monster from attacking Hart. Eventually, Grendel decides to kill Wealtheow, since she threatens the ideas explained by the dragon. Upon capturing her, he realizes that killing her and not killing her would be equally meaningless acts, and he retreats, knowing that by not killing Wealtheow, he has once again confounded the logic of humanity and religion.
Later, Grendel watches as Hrothgar's nephew Hrothulf develops, through dialogue with a peasant named Red Horse, his understanding of the two classes in Danish society: thegns and peasants. Red Horse incites in Hrothulf a revolutionary spirit.
Grendel watches a religious ceremony and is approached by an old priest named Ork, who thinks that Grendel is their main deity, the Destroyer, and engages him in conversation. When three other priests approach and chastise Ork, Grendel flees, overwhelmed with a vague dread.
Watching the Danes, Grendel hears a woman predict the coming of an illustrious thegn and then witnesses the death of the Shaper. Returning to his cave, his mother seems agitated. She manages to make one unusual unintelligible word, which Grendel discounts, and then goes to the Shaper's funeral. Later, in the cave, he wakes up with his mother still making word-like noises, and once again feels a terrible foreboding.
Grendel reveals that fifteen travellers have come to Denmark from overseas. The visitors, who reveal themselves to be Geats ruled by Hygelac, have an uneasy relationship with the Danes. Upon their arrival, Grendel notices the firm nature of their leader, Beowulf, and the fact that his lips do not move in accordance with his words, and sees a great lust for violence in Beowulf's eyes, indicating to Grendel that he is insane.
At nightfall, Grendel attacks. When he believes that all the men are sleeping, he breaks into the hall and eats one man. The second man he grabs turns out to be an alert Beowulf. They wrestle furiously, during which Beowulf appears to become a flaming, dragon-like figure and repeats many of the ideas that the dragon espoused to Grendel. Beowulf rips off Grendel's arm, causing the monster to flee. Grendel proceeds to toss himself into an abyss (whether or not Grendel jumps is left up to the perception of the reader), and dies wondering if what he is feeling is joy.
