The three stars are for Jean Peters. Samuel Shellabarger's novel was not his best (that honor goes to "The King's Cavalier"), but it deserved better treatment than this Hollywood hack job. The only thing that redeems this terrible picture is the irresistibly gorgeous Jean Peters (whose acting is fine, although it can't hope to measure up to her looks).
Major flaws that put this picture beyond saving:
1) Tyrone Power terribly miscast as Pedro de Vargas. He's just way too old! In Shellabarger's book, Pedro begins the story as a callow 19-year-old. He is just beginning to feel his oats as a man; he is completely untried as a soldier and swordsman. He is a virgin who worships Luisa de Carvajal, stuttering as his heart leaps into his throat when he gets a chance to speak to her.
To be fair, it may have been impossible to cast this role in 1947. The character needs to grow from an innocent, impetuous youth, through torture, attempted murder, near-death experiences, war, the loss of a child - until he becomes a mature gentleman, lover and soldier. Even today's young actors, who accept such roles much earlier in life than they would have in Power's day, would be hard-pressed. Perhaps Heath Ledger or Orlando Bloom (with a few extra acting classes for each of them) could manage it. (Pedro is supposed to be slim and light-haired, as well as young.)
2) As mentioned elsewhere, the ending of the picture halfway through the story. This leaves out the comeuppance of de Silva and de Lora, as well as the true return of the Coatl character, precisely when he's needed.
3) The character of de Lora, the evil Inquisitor, is almost completely omitted. Without his presence, much of Juan Garcia's motivation disappears. The auto da fe scene is also omitted - a scene which (along with the causeway battle scene, also left out!) is one of the finest set pieces in the novel.
4) The picture shouldn't have been made in the first place. The novel is mostly episodic; it is perfect for a miniseries - a format that could remedy many of the omissions. I'd suggest four two-hour episodes.