The basic plot derives from Beowulf, a heroic poem of unknown authorship written in Old English and preserved in a manuscript dating from around AD 1000. The poem deals with the heroic exploits of the Geat warrior Beowulf, who battles three antagonists: Grendel, Grendel's mother, and, later in life, an unnamed dragon. Gardner's retelling, however, presents the story from the existentialist view of Grendel, exploring the history of the characters before Beowulf arrives. Beowulf himself plays a relatively small role in the novel, but he is still the only human hero that can match and kill Grendel. The dragon plays a minor part as an omniscient and bored character, whose wisdom is limited to telling Grendel "to seek out gold and sit on it"; his one action in the novel is to endow Grendel with the magic ability to withstand attacks by sword (a quality Gardner found in the original).
Gardner himself explained that his Grendel character is modeled on Jean-Paul Sartre, with whom Gardner claimed to have a love-hate relationship: "he's a horror intellectually, figuratively, and morally, but he's a wonderful writer and anything he says you believe, at least for the moment, because of the way he says it....What happened in Grendel was that I got the idea of presenting the Beowulf monster as Jean-Paul Sartre, and everything that Grendel says Sartre in one mood or another has said".