The story is adapted from books IX and X of Geoffrey of Monmouth's History of the Kings of Britain, but contains numerous episodes that are not in Geoffrey's work, such as the Round Table. Some parts do not have a clear source and may have originated with the poet.
Tthe Alliterative Morte Arthure is a relatively realistic version of events. There are few of the fantastical elements which often surround the legend and the story focuses more on Arthur's skill as a warrior king. It contains little of the magic, with no mention of Merlin, although it does use the literary device of the dream vision common in courtly romancer. Arthur is a more political and also flawed ruler, the story is not just based in a small realm but is always placed within a wider European situation and this Arthur is more clearly Christian than other versions. Arthur also has two legendary swords, the first being Excalibur (referred to as Caliburn, an earlier name of the sword), and the second one being Clarent, a formal sword, stolen by the evil Mordred, with which Arthur receives his fatal blow close to the banks of the Tamar.
An example of the differing style of the alliterative version of the story is the treatment of Mordred. He is not simply the villain of the piece as he is in other poems but is a complex character with a varying personality. One mark of the prevalence of Christian morality in the poem is that even Mordred cries and seems to be repentant around line 3886. This is also one of the only versions of the legend where Guinevere has children, albeit with Mordred.