The novel is inspired by a Welsh tradition that King Arthur had not one but three different queens, all named Gwenhwyfar (Guinevere). The protagonist of the novel is the third and youngest. Lackey also follows the Welsh tradition about Gwenhwyfar having a sister named Gwenhwyfach.
In the novel, Gwenhwyfar (nicknamed Gwen) was born as the third of four princesses to a Celtic king. Her mother has noticed that she has powerful potential for magic, but Gwen would rather spend her time with horses. Her father, who has no sons, encourages her to pursue her dreams, and Gwen becomes a warrior. Meanwhile, the High King Arthur is trying to unite his kingdom while striking a delicate balance of power between the Ladies and Druids of the Old Ways, and the newly arrived Christian priests.
The story follows Gwen from childhood all the way to adulthood, telling the stories of both her life, and the part she would play in Arthur's.
This work provides examples of:
- Action Girl: Gwenhwyfar, obviously. Her childhood idol, Braith, also qualifies. The original Gwenhwyfar, Arthur's first wife, is likewise mentioned as having been skilled with a bow.
- Adaptational Heroism: Melwas is still a usurper, but isn't a kidnapper or rapist like in most versions of the myth since it's revealed that the second Gwen is with him willingly.
- Anything That Moves: Anna Morgause, apparently. While some of it might be exaggerated, she doesn't think anything of screwing her own brother for power.
- Big Bad: In the first part it's Anna Morgause and Morganna for their attempts to enchant Gwen's father, but when Medraut grows up he eventually takes center stage as the one who personally targets Gwen.
- Blood Magic: Morganna, Anna Morgause, and Gwenhwyfach seem to use this sort of magic, killing black cats and other animals for power. The magic used by the castle ladies isn't without sacrifice, but it's treated more as a grim necessity.
- BrotherSister Incest: Arthur and Anna Morgause, as per the legend.
- Charm Person: Gwenhwyfach has this power. It makes her spiteful, cruel, and extremely entitled. Arthur is also one, even more so because he does have the strength and wisdom his glamour projects. She attempts to charm him, but he ends up charming her.
- The Chessmaster: Morganna has shades of this, though she's barely present in the story. Her nephew, Medraut, however, has numerous schemes and backup plans in the works.
- Creepy Child: Medraut is mentioned as having been rather unnerving as a boy. Little Gwen, also had shades of this, especially as she became more aware of her ability to control men.
- Damsel in Distress: During the final third of the book, Gwenhwyfar is kidnapped by the Big Bad and replaced with an impostor. Subverted in that Gwen takes advantage of a distraction to escape Medraut's tower, and ends up saving Lancelin— who had come to rescue her.
- Enfante Terrible: Even as a baby, Medraut is distinctly off. The first time he and Gwen meet, he can't be more than five, but is already deciding that he will own her one day.
- Deal with the Devil: Subverted. Gwen's deal with the Annwn is actually favorable for all parties involved.
- Death of the Old Gods: As in many Arthurian stories, the displacement of the native pagan religion by Christianity is a constant presence. It's ultimately subverted in the epilogue: Gwen realizes that the ladies of Avalon are hiding pagan traditions within Christianity, so even though their power wanes the religion will live on.
- Everyone Hates Hades: Played with. Gwen is definitely afraid of the Morrigan, but she doesn't consider her evil.
- The Fair Folk: The Annwn can be absolutely merciless and are renowned tricksters. Fortunately for Gwen, they owe her a debt, and Gwyn ap Nudd treats her with the utmost respect
- Faking the Dead: Arthur's second wife faked her death and went to live in the monastery.
- Femme Fatale: Anna Morgause's main tactic is seduction, most infamously used on her own brother. Since she's the first example of female sexuality Gwen encounters, it leads to Gwen having an aversion to sex for most of her earlier years until she grows up enough to realize that it wasn't sex itself that was bad, but Morgause using it as a weapon.
- Four Is Death: Gwenhwyfach, or "Little Gwen", was the fourth daughter and Gwenhwyfar's little sister. She is also the pettiest, cruelest, and most manipulative of the four sisters.
- Genre Savvy: The abbot, Gildas, realizes that Gwen has been replaced with an impostor almost immediately, as did Lancelin. Arthur, on the other hand, only learned what was going on when he was called out for not figuring it out seven months later.
- I Have You Now, My Pretty: When Medraut was a child, he announced his intention to marry Gwen... then he kidnaps her from Arthur's court many years later.
- Lie Back and Think of England: Since neither wanted to be married, the only reason Arthur and Gwen have sex is to conceive a son for the good of the realm and they both hate it. Gwen mostly just endures it until it's over, while Arthur is routine and clinical and treats Gwen like a sex doll rather than a partner. When Gwen makes love to Lancelin, she's surprised that sex can actually be fun.
- Marry for Love: Arthur with the second Gwenhwyfar. After she betrayed him, he loses all interest in love, marrying Gwen mostly for the horses her father offered as dowry and treating her like a breeding mare rather than a wife.
- Nice Job Breaking It, Herod: Arthur and the Merlin conspire to kill all the newborn infants in the kingdom at one point in an effort to kill Arthur's illegitimate son (and nephew), Medraut
- No Guy Wants an Amazon: Gwen is warned that few men will be able to see her as both a woman and a warrior. And indeed, this seems to be true (especially where Arthur's concerned) Lancelin turns out to be able to see her as both, after some development on his part.
- Odd Friendship: Gwen, a devout follower of Epona, and Gildas, a devout abbot. He originally scorns her as a devil and she scorns him as small-minded, but they eventually realize they've got a lot in common.
- One-Steve Limit: Notably averted. Arthur is married to three different women named Gwenhwyfar over the course of his reign, though the third one is the only one who gets much focus. If you count Gwenhwyfach, there are four Gwens.
- Real Women Don't Wear Dresses: Deconstructed. Gwen is warned that the idea of being a woman and a warrior is mutually exclusive in the minds of many, so if she wants to be respected as a warrior she can't ever be seen as womanly. Bronwyn later chides her for thinking this way about her female spies who are more visibly feminine. And then outright inverted when she and Gildas became friends because she treated his priests with respect and honesty, which they don't get often because they are perceived as "womanly".
- Scarecrow Solution: The 'white spirit' in the title is the meaning of Gwen's name. It inspires her alter-ego, the guise of a vengeful spirit that she uses to terrify Saxons with the help of a little misdirection and some white chalk on her face.
- Self-Serving Memory: Morgana. She told Medraut that when she went to marry Gwen's father, she scorned him for being too old for her. The truth was, she and Morgause did their best to charm him, but the castle ladies turned the magic back on them.
- Spell My Name with an S: Most of the characters have names that are different than how most people would recognize them: Gwenhwyfar instead of Guinevere, Medraut instead of Mordred, Lancelin instead of Lancelot... In most cases, this is because the author is using the original Welsh spelling. Not for Lancelot though, as he was most probably invented by French authors.
- Spoiled Brat: Gwenhwyfach constantly torments her sisters or just anyone near her, throws awful tantrums whenever someone else gets something they like, and seems dedicated to making everyone miserable. Unusually for this case, it's no fault of her parents, who do their best to discipline her. It just doesn't take.
- Sympathetic Adulterer: Arthur is misogynistic and dismissive of Gwen and basically treats her as a broodmare for a future child. Sleeping with Lancelin, who actually loves and respects her, is liberating and empowering for her.
- What Happened to the Mouse?: Morganna just seems to drop off the face of the earth halfway through the book.
- What the Hell, Hero?: Gwen slaps Arthur and shames him in front of the entire Round Table for falling for Medraut's "fake Gwen" trick and for his Stay in the Kitchen attitude toward her despite her status as a warrior, then firmly tells him she's going to war whether he likes it or not. This finally gets through to him and he addresses her as War-chief Gwenhwyfar for the first time.