Azalea visits her mom before the Yuletide Ball that her dad, the King, throws every year. Mom is sick, and makes Azalea swear on her silver handkerchief to take care of her sisters; and then the Ball is ruined when Mom dies during it. Azalea really does have to take care of her younger sisters now—all eleven of them. (Yikes.) Their dad, the King, grows increasingly distant after their mother dies, and because the household is in mourning, the girls are forbidden from dancing.
Luckily Azalea discovers a magic passage in their room though, that leads to an enchanted forest with a pavilion in it. The keeper of the pavilion—who goes by the super original name of Keeper—invites them to come every night to dance there. The girls promise to keep this place a secret so no one can take it from them, and they even swear on their mom's silver handkerchief. Cool, right? Not so fast.
As it turns out, swearing on silver is Serious Business. So in exchange for dancing there, Keeper makes the girls go looking for a magic object that will free him from the pavilion. And when things turn nasty with Keeper—turns out he's the old mad High King, who was totally sadistic and revenge-driven—they can't ask for help… because they swore themselves to secrecy. Oops.
In the meantime, the King goes away to war, and comes back again. Azalea begins to get acquainted with Mr. Bradford, whose father was in the parliament, but things get awkward when he thinks she's one of her sisters. The King invites gentlemen to the palace to solve the riddle of where the princesses go dancing every night, but it's really just a way for Azalea to start meeting eligible bachelors, which would otherwise be forbidden during mourning. Way to bend the rules for yourself (but not your daughters), Your Highness.
Anyway, one of these bachelors who comes by to solve the riddle is Lord Teddie, a dude from a nearby country who alternately annoys and charms the girls. Another is Fairweller, the prime minister, who is so serious that literally no one seems to like him.
Things come to a head when the girls run out of time to find the magic object, and as revenge Keeper traps the eleven younger sisters in mirrors that will eventually kill them. Azalea destroys her mother's silver handkerchief, which was the magic object keeping Keeper confined, and he enters the rest of the palace and starts placing hostile magic everywhere.
Then stuff gets really wacky. Azalea has a vision of her dead mother, who gives her the handkerchief again and makes her promise to look out for all her family—this means her father as well as her sisters. The handkerchief somehow reappears in Azalea's hands, and she uses it to get her sisters out of the mirrors; then the King, Mr. Bradford, Lord Teddie, Fairweller, and other dudes ride to the rescue.
Somehow Keeper gets a gun and manages to shoot the King, thereby fulfilling his oath for revenge—and because the oath was the thing magically keeping him around all this time, he now disappears. See ya, Keeper. The twelve girls all cluster around their father's body, and a deep, nameless kind of magic fills them with warmth, bringing him back to life.
In the next few months, Lord Teddie asks for Azalea's sister Bramble's hand, Fairweller sorta gets permission to eventually court the next youngest sister, Clover, and Azalea waits and waits to hear from Mr. Bradford. Turns out he's been going through the proper channels to officially marry her and get involved in politics through parliament. Way to be on the up and up, Mr. B. Azalea and her sisters reconcile with their father, welcome Mr. Bradford to the family, and everyone is happy and able to dance again. Good times.