Grendel is a large, frightening monster who lives in a cave near King Hrothgar's castle. Though he looks like a beast, he is intelligent. He can speak well, tell sophisticated jokes, and theorize about the meaning of life. He tells his stories in a series of flashbacks, mixed in with "present-day" scenes. He lives with his mother, who he hates for her apparent inability to speak or reason, and her sometimes suffocating love. He hates the animals around him for their animal stupidity. He even hates the sky above him, because it ignores him. He happily eats Hrothgar's people, enjoying their screams. He remembers when he first found the outside world: he dove through a secret pit in his cave, and wandered around until he got his foot caught between two trees. As he cried for his mother, he was approached by a group of men who, for no real reason, attacked him. His mother fought them off. This was his first meeting with Hrothgar. He remembers Hrothgar's development as a king: when all the other minor rulers were fighting each other, Hrothgar had the sense to make friends with his neighbors, slowly convincing them to be part of one large kingdom ruled by him. A singer (Grendel calls him the Shaper) came to the castle, and awed everyone with his beautiful music and his songs about the glorious history of their people. Grendel was deeply moved, but at the same time he believed that the singer had just put a good face on battles that were, in reality, brutal and petty.
Grendel says he has changed the singer's songs: now they are full of sadness, because of all the death he has witnessed. Grendel remembers that listening to the songs, and the confusion they caused in him, made him aware of some other presence. He finally gave into it, and visited the dragon, who had apparently been beckoning him from his lair. The dragon was a frightening beast, even for Grendel. He enjoyed cruelty, and believed life was pointless. Grendel tried to talk to him about the Shaper, but the dragon dismissed the songs as an attempt to create meaning and community where there was none. He told Grendel to do whatever he wanted, because there was absolutely no reason not to. Though Grendel is confused by this advice, and doesn't entirely believe it, he is influenced by the dragon. The feelings of rage and isolation he has always had are finally supported. He has a new confidence in his animal need to kill. Plus, the dragon has enchanted him, so that he can't be hurt. This forces him further into isolation--he doesn't even have to fight the men before he eats them--but he also enjoys it. Unferth, one of the strongest men, approaches him. When he learns that Grendel is invincible and can speak, Unferth begins to talk grandly about how he, a hero, will defeat the evil Grendel. Grendel laughs at him, making it clear that he thinks heroism is a sham, then knocks him to the ground and leaves. Incredibly, Unferth follows him, crawling, and arrives at Grendel's cave days later. He tries to explain how noble his heroic acts are, but Grendel just smirks, and decides to punish him by refusing to kill him. For years after, Grendel protects Unferth, even though he wants desperately to die a hero's death.
Grendel is becoming a little bored. He thinks about killing the queen. She is a beautiful, pure young woman who was given to Hrothgar as a peace offering. Grendel is confused by his feelings for her: he sees something meaningful in her beauty and kindness, and he hates himself for it. He decides to kill her, and nearly does, but then decides that to kill her would be just as pointless as letting her live. Soon after, Hrothulf, Hrothgar's nephew, comes to live with them after his father is killed. Hrothulf is a sullen, thoughtful teenager, who believes that the peasants should not tolerate their poverty while the king is so rich, and also covets the throne. He has long talks with an old, anarchist peasant, who serves as his mentor and advisor.
Grendel feels like something bad is going to happen soon, but he doesn't know what. He waits. He toys with a priest, pretending to be God and asking the old man to describe Him. But the priest surprises Grendel: he gives a thoughtful analysis of religion, and then is moved to tears, thinking he has finally achieved his life's goal. Grendel leaves in angry confusion. Later, he is still waiting, increasingly nervous. He tries to kill a mountain goat, but it will not stop climbing toward him--it is, he says, the goat's nature to climb, so it climbs. The Shaper dies, and Grendel is pleased. This changes things: he is less reluctant to kill the townspeople now. His mother seems to be warning him of something, but he doesn't understand her. He goes to the Shaper's funeral, knowing that his world is changing.
Finally, something has happened. Strangers from across the sea have come: huge men, the largest of whom (Beowulf) promises to kill Grendel. Grendel is very excited: this man is strange and frightening, but he also looks forward to fighting with him. There is a long day of anticipation, and then night falls. Grendel breaks into the meadhall, and everyone is asleep. He eats one man and then picks up another. But this one was only pretending to sleep--it is Beowulf, and he has the strongest grip Grendel has ever felt. He rips Grendel's arm off, and Grendel runs away, saying it was an accident--he was tricked. He knows he is dying and somehow cannot stop himself from running towards a dark pit. The animals he has tormented over the years watch as he falls into it, crying bitterly.
