The work starts out with a lengthy prose section describing the circumstances leading up to Grímnir's monologue, which comprises 54 stanzas of poetic verse. The last bit of the poem is also prose, a brief description of Geirröth's demise, his son's ascension, and Odin's disappearance. It should be noted that the prose sections were most likely not part of the original oral versions of Grímnismál.
The narrative commences at a point when Odin and his wife, Frigg, were sitting in Hlidskjalf, looking out on the worlds. They turned their eyes towards King Geirröth, who was reigning in the stead of his late father, King Hrauthung. Geirröth and his older brother Agnarr had been raised by Odin and Frigg, respectively. The god and goddess disguised themselves as a peasant and his wife, and taught the children wisdom. Geirröth returned to his father's kingdom where he became king upon his father's death, while Agnarr dwelt in company with a giantess in a cave.
In Hliðskjálf, Odin remarked to Frigg that his foster-child Geirröth seemed to be prospering more so than her Agnarr. Frigg retorted that Geirröth was so parsimonious and inhospitable that he would torture his guests if he thought there were too many of them. Odin disputed this, and the couple entered into a wager in this respect. Frigg then sent her maid Fulla to Geirröth, advising him that a magician would soon enter his court to bewitch him, and saying that he could be recognised by the fact that no dog was fierce enough to leap up at him.
Geirröth heeded Fulla's false warning. He ordered his men to capture the man the dogs wouldn't attack, which they did. Odin-as-Grímnir, dressed in a dark blue cloak, allowed himself to be captured. He stated that his name was Grímnir, but he would say nothing further of himself.
Geirröth then had him tortured to force him to speak, putting him between two fires for eight nights. After this time, Geirröth's son, Agnarr, named after his brother, came to Grímnir and gave him a full horn from which to drink, saying that his father, the king, was not right to torture him.
Grímnir then spoke, saying that he had suffered eight days and nights, without succour from any save, Agnarr, Geirröth's son, whom he prophesied would be Lord of the Goths. He then revealed himself for who he was, as the Highest One, promising him reward for the drink which he brought him.
In the body of the poem, Odin describes at great length the cosmogony of the worlds, the dwelling places of its inhabitants, and talks about himself and his many guises.
Eventually, he turns to Geirröth and promises him misfortune, revealing his true identity. Geirröth then realized the magnitude of his mistake. Having learned that he is undone, he rose quickly to pull Odin from the fire, but the sword which he had lain upon his knee slipped, fell hilt down, the king stumbled and impaled himself upon it. Odin then vanished, and Agnarr, his son, ruled in his stead.