Grendel begins with the title character engaged in a twelve-year war against the human Danes. 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 which he is unable to speak. One day, however, he arrives at a pool filled with firesnakes, which he enters. Upon exiting, he is greeted by moonlight. Exploring the mysterious outside world at greater length, he eventually becomes wedged and trapped in a tree. Helpless, he cries for his mother, but only a bull appears, wounding him. The bull's unchanging, unrelenting manner of attack leads him to conclude that the whole of reality is tantamount to the animal's senseless efforts (a nihilistic view). As he is able to evade its blows, he falls asleep, only to wake surrounded by humans. The armored men, thinking that he is a tree spirit, try to feed him. Although Grendel can understand the humans, they cannot understand him and they become frightened, which leads to a fight between Grendel and the Danish warriors, including Hrothgar. Grendel is barely saved from death at the hands of the humans by the appearance of his mother.
The novel continues by elaborating on the colonization of the area by humans and their subsequent development from nomadic bands into complex civilizations with fine crafts, politics, and warfare. Grendel witnesses Hrothgar become the foremost in power amongst the human factions. During Hrothgar's rise to prominence, a blind poet appears at the doors of Hart, whom Grendel calls "the Shaper".[b] He tells the story of the ancient warrior Scyld Shefing, which enraptures and seduces Grendel. The monster reacts violently to the power the beautiful myth has on him and flees, having seen the brutal rise of the Danes. Grendel continues to be enraptured by the tales, as does Hrothgar, who begins a widespread campaign of philanthropy and justice. After seeing a corpse and two lovers juxtaposed, he drags the corpse to Hart, bursting into the hall and begging for mercy and peace. The thegns do not comprehend his actions and see this as an attack, driving him from the hall. While fleeing the men, he curses them, yet still returns later to hear the rest of the Shaper's songs, half enraptured and half enraged.
When Grendel returns to his cave, he attempts and fails to communicate with his mother, thus leaving him with a sense of total loneliness. He becomes filled with despair and falls through the sea, finding himself in an enormous cave filled with riches and a dragon. The omniscient dragon reveals to Grendel a totally fatalistic view of reality. The dragon explains the power of the Shaper as simply the ability to make the logic of humans seem real, despite the fact his lore possesses no factual basis. The dragon and Grendel cannot agree about the dragon's statements that existence is a chain reaction of accidents, and Grendel exits the cave in a mixed state of confusion, anger, and denial.
While listening to the Shaper, he is spotted by sentries, who try to fight him off again, but he discovers that the dragon has enchanted him, leaving him impervious to weapons. Realizing his power, he begins attacking Hart, viewing his attacks as a perpetual battle. Grendel is challenged by a thane named Unferth, to which he responds mockingly, leaving when Unferth runs away crying. 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, leading the Dane to threaten Grendel with death, in the hope that his people would sing of his tale for years to come. When Unferth passes out from exhaustion, Grendel takes him back to Hart to live out his days in frustrated mediocrity.
In the second year of the war, 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, keeping the monster from attacking Hart just as she prevents internal conflicts among the Danes. Eventually, Grendel decides to kill Wealtheow, since she threatens the ideas explained by the dragon. Upon capturing her, he realizes that killing and not killing are equally meaningless, 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 his understanding of the two classes in Danish society: thegns and peasants. He wrestles with his anarchist theories and then further explores them with a peasant named Red Horse, who teaches Hrothulf that government exists only for the protection of those in power. As the politics of Hrothulf, Hygmod, Hrothgar, and a thegn named Ingeld become more bitter and pathetic, Grendel defends his terrorizing of the Danes, claiming that his violence has resulted in great deeds and given the people humanity, thus making him their creator.
While there had previously been foreshadowing of the death of Grendel, the character himself begins to feel an uneasy sensation that becomes fear. Grendel then watches a religious ceremony and considers the futility and role of religion. While sitting in the circle of the Danish gods, an old priest, Ork, approaches the monster. Thinking that Grendel is their main deity, the Destroyer, he talks to Grendel, who plays along, questioning Ork. The priest explains a theological system that borders on monotheism, bringing him to tears. While Grendel is puzzled by the fervent belief, three other priests approach and chastise Ork. Grendel flees at this opportunity, overwhelmed with a vague dread.
Grendel again fights an animal in his lair, but gives up after even death will not stop its mechanical climb. Watching the Danes, he 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. The Shaper's assistant sings a song derived from the tale of King Finn (see the Finnsburg Fragment). 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 over the sea, almost as though the way was set before them. He has a morbid exhilaration from these visitors, most especially from their huge and taciturn leader. The visitors, who reveal themselves to be Geats ruled by Hygelac, have an uneasy relationship with the Danes. Upon their arrival, Unferth mockingly claims that the leader of the visitors has lost a challenge to another champion. The Geat leader, Beowulf, calmly relates his version of the events, and then rebukes Unferth, who leaves on the verge of tears. Grendel notices the firm nature of Beowulf and the fact that his lips do not move in accordance with his words, as though he is dead or risen from the dead. He sees a great lust for violence in Beowulf's eyes, convincing Grendel he is insane.
At nightfall, Grendel attacks. He breaks into the hall and eats one man. Grabbing the wrist of another, he realizes that it is Beowulf, and that he has grabbed his arm. They wrestle furiously, during which Beowulf appears to become a flaming dragon-like figure and repeats many of the ideas that the dragon revealed to Grendel. As Beowulf gains the upper hand, Grendel tells himself that were it not for a slip on a puddle of blood, Beowulf would not be in control of their battle. The Geat slams Grendel into the walls of the hall, demanding that Grendel sing about the hardness of walls. This is a continuation of Grendel's poetic exploration of philosophy. He then rips off Grendel's arm, causing the monster to flee in pain and fear. Grendel feels as though everything is unnaturally clear, leading him to toss himself into an abyss (whether or not Grendel jumps is left up to the perception of the reader). He notes as he dies that the only creatures attending his "funeral" are the animals he so despised. Grendel dies wondering if what he is feeling is joy, understanding what the dragon meant by the accident statement, and cursing existence.