The setting is vaguely Arthurian. Valiant himself is a Nordic prince (from the faraway Thule - apparently, it is located somewhere on the Norwegian coast). Early in the story, Valiant comes to Camelot and becomes fast friends with Sir Gawain, Sir Tristram (see Tristan), King Arthur and Merlin, and becomes a Knight of the Round Table. Later, he meets the love of his life - Aleta - on a Mediterranean island. He fights the Huns with his magic Singing Sword, Flamberge, he travels to Africa and to America and he helps his father regain his lost throne of Thule.
The historical and mythological elements of Prince Valiant were initially chaotic, but soon Foster attempted to bring the facts into order. Some of the elements of the story (for instance, the death of Attila the Hun in 453, the murder of Aetius in 454, though different from the historical version (Valiant and Gawain receive are blamed for the murder and must flee), and Geiseric's sacking of Rome in 455, which Prince Valiant and Aleta witness), place the story in the 5th century. Some slightly fantastic elements, like "marsh monsters" (a dinosaur-like creature) and witches, are present in the first volumes but are later downplayed (as are Merlin's and Morgan le Fay's magicks), so that by the fifth album, the story is in most aspects a realistic one not only in drawing style but also in content. That of course does not mean it is true or believable.
The first panels of Prince Valiant were published in 1937 and the comic was continually published until the death of the author in 1982. From 1970 on, Foster illustrated half the pages, and John Cullen Murphy the other half. In 1978, Foster stopped drawing, and from then on only wrote the manuscript (illustration was done entirely by Murphy during those years). The full stretch of the story is some 1800 panels.
Since Foster's death, Murphy has assumed both writing and illustration. He has moved the setting of the narrative forward in time (e.g, the Byzantine Emperor Justinian has repeatedly appeared as a villain, threatening Aleta's realm "The Misty Isles").
Chaosium produced a role-playing game with the name Prince Valiant.
Movie adaptations
External links:
http://www.bpib.com/illustra2/foster.htm - material about Foster, but also includes some of Foster's illustrations.