Saving King Arthur! Night of the Ninth Dragon by Mary Pope Osborne

"Oki! Wait!" shouted Annie. "Stay!"

But the puppy kept running. He crossed the street and disappeared into the Frog Creek woods.

Jack and Annie raced into the woods after their puppy. "You were right!" wailed Annie. "We should have put him on the leash!"

"Yip, yip!"

They found the puppy at the base of a giant oak tree. The magic tree house was nestled high in the treetop.

Jack and Annie race up their rope ladder into the tree house, expecting to find the usual guidebook and rhyming challenge from Morgan Le Fay or Merlin. But there's nothing. Then Oki begins to bark at a piece of paper blowing toward the ground. Annie scurries down the ladder and retrieves it. It is a brief unsigned note in old-fashioned handwriting:

Dear Jack and Annie,
     Please come to Camelot.

Jack and Annie have never found such a vague message before. But they can't turn down a chance to visit the legendary city of Camelot, so Annie points at the word Camelot and their tree house begins to lift off and spin, and then everything becomes still and shadowy.

The tree house rests in the gnarled branches of an ancient tree in a wood. That's not unusual. But Jack and Annie are perplexed. What is most unusual is that they are still dressed their regular jeans and tee-shirts, not the medieval clothes they had worn in Camelot before. And there's no sign of the spires of Camelot in sight. But following Oki who pulls them steadily on his leash, they soon see the walls of Camelot with King Arthur's flags with the dragon insignia blowing in the wind.

But as they approach the gates, they are met with a disturbing sight--Queen Guinevere herself, appearing distraught and even disheveled, running out to meet them. And her story alarms Jack and Annie.

War has come to the kingdom of Camelot, and King Arthur has been mortally injured in battle and lies inside in a secret chamber in the castle, kept perilously alive only by a dwindling supply of magical waters from Avalon. The only hope for saving Arthur and the kingdom of Camelot is to get him to Avalon's curative lake and find the stolen golden Ninth Dragon icon and return it to Morgan LeFay's secret garden to restore the Merlin's protective spell against their enemies over Camelot.
Disguises as peasants in ragged hooded cloaks, Jack and Annie, Guinevere, and King Arthur set out in an ox cart for the cottage of a a famous seer, Cafelle, whose vision comes in the form of a riddle for Jack and Annie:

Moon so bright,
Munith Mor night.
Curtain of white,
Hides from sight.
Magic for flight,
Before dawn's light,
Or lose the fight."

Cafelle's vision tells them to follow the cart path across the moor, and with the supply of magical waters running low and King Arthur growing dangerously weak, Jack and Annie set forth toward Avalon. But suddenly, the oxen's harness breaks in the middle of nowhere.

"We have to keep going."

"Never give up!" said Jack.

Jack takes a place behind the cart to push, and Annie and Queen Guinevere take up the wagon's tongue and begin to pull the cart toward Avalon.

Will Jack and Annie's gift for solving riddles save King Arthur and Camelot? As the past 54 books in this series suggest, Jack and Annie will come through, in Mary Pope Osborne's latest, Night of the Ninth Dragon (Magic Tree House (R) Merlin Mission) (Random House, 2016), a series which celebrates the ordinary powers of imagination and perseverance that Jack and Annie together have. Osborne's best-selling beginning chapter books provide plucky young heroes and a taste of folklore, history, and geography that combine fantasy adventure and plenty of incidental learning along the way, all with the help of the inspired black and white illustrations of Sal Murdocca. For kids who want "just the facts," along with their magic, pair this one with its companion Fact Finder volume by Natalie Pope Boyce, Dragons and Mythical Creatures (Magic Tree House (R) Fact Tracker).