The final battle has arrived!
The combined armies of the Nome Axis march through the tunnel toward Oz. As they march, the Nome King and General Guph do a little scheming of their own. It turns out Guph has given some thought to ways of dealing with the terrifying allies he has recruited, and he explains his plan to the Nome King: while the Whimsies, Growleywogs, and Phanfasms are conquering Oz, the Nome King must recover his Magic Belt as quickly as possible. Then he can use it to wish the other armies back to their homes, leaving the Nomes in sole possession of Oz and all its treasures.
The only problem now is the dust. It seems the tunnel is terribly dusty. As the armies march along all the invaders are plagued by dust in their eyes, noses, and throats. All of them become very thirsty.
Apparently King Roquat, like many would-be conquerors, has no sense of logistics at all. (To be fair, it may be less of a concern to immortal beings with magic powers.) He hasn't provided drinking water for his armies. Apparently nobody has so much as a hip flask.
If the invasion is that slapdash, maybe the Soldier With The Green Whiskers could defeat it after all.
As dawn approaches, the Scarecrow, the Tin Woodman, Jack Pumpkinhead, Tiktok, Ozma, and the rest all gather in the garden where the invaders will soon break out. Ozma has been watching the armies on her Magic Picture. The villains are having a terrible time because of the dust -- and the Scarecrow reveals that it was Ozma who generated all that dust by means of the Magic Belt.
It's H-Hour. The ground breaks open and the First and Foremost of the Phanfasms climbs out, followed by all his terrible warriors. The kingdom of Oz lies helpless before them. The triumph of evil is at hand . . .
. . . but first they want a drink after all that marching through a dusty tunnel. Fortunately, there's a lovely, cool Forbidden Fountain right there.
All the invaders following the Phanfasms make the same mistake. They rush for the Fountain, drink deep, and then forget why they've come. The only one who isn't affected is the Nome King himself, because his underling General Guph rudely elbows him aside to get at the water, and Roquat is so upset by this act of lese-majeste that he forgets his thirst. The Scarecrow and Tin Woodman grab him and throw him into the Forbidden Fountain so that he, too, will forget who he is and why he came.
Ozma politely suggests to the Nome King that he order his soldiers to march home, and he agrees, happy to be obeyed so readily by the Nomes and delighted to be able to do a favor for the lovely Girl Ruler of Oz. The Magic Belt sends the rest of the invasion force back whence they came, and Oz is safe.
Or is it?
Remember, this is 1910. The Wright Brothers have been flying around since 1903, and Count Zeppelin is already working on his fifth dirigible airship. Simply being surrounded by a Deadly Desert isn't enough to protect Oz any more.
Ozma and her court head south to consult with Glinda the Good. The Girl Ruler of Oz never does anything without consulting Glinda, and she always follows Glinda's wise advice. That's how Ozma remains Girl Ruler of Oz. Presumably because if she stopped following Glinda's advice something unfortunate might happen.
Since Glinda has a Magic Book which informs her of everything that happens everywhere (which means she knew all about the invasion plan and doubtless was preparing her own defenses), she knows why Ozma has come to call upon her.
Glinda agrees that Oz is at risk of being discovered by people from the outside world. Fortunately she has devised a powerful magic spell to make the entire country invisible to the eyes of outsiders, so that no one will ever be able to locate Oz again. In fact, so eager is Glinda to serve her lovely Girl Ruler that she went ahead and cast the spell before Ozma even reached her castle.
She just lives to serve, doesn't she?
Presumably at this announcement Dorothy and Agent Diggs (a.k.a. the Wonderful Wizard) exchange significant glances. They're cut off from any help from President Taft -- but presumably Glinda's spell also means the threat from European or Japanese operatives in Oz has been eliminated. For now, they must remain as agents in place.
Dorothy does get one message out, to the Royal Historian of Oz, L. Frank Baum himself. Baum reports sadly that he won't be able to tell any more stories about the marvellous land of Oz now that it is cut off magically from the rest of the world.
He's got another eight books' worth of Oz in him. His readers aren't going to let him stop, not while he's still alive. L. Frank Baum can struggle against his fate, but there's no way he can escape from Oz.
Now, while the Scarecrow's plan to thwart the invaders by means of the Forbidden Fountain is certainly clever, it does raise some questions. Ozma refuses to fight for her country, but is perfectly willing to use the Magic Belt to fill the tunnel with dust so the invaders will drink the Water of Oblivion and forget.
So why not use the Belt to make them forget as soon as they start out? Or use it to bend the tunnel into a closed loop so that the armies will march forever and never come to their breakout points? (She did something very similar to a Kansas highway in Road to Oz, so it's obviously within the Belt's power.)
Given the powers at her disposal, it becomes obvious that suckering the invaders into drinking the Water of Oblivion was Ozma's goal all along. The Forbidden Fountain not only prevented the invasion, it destroyed the minds of three powerful and aggressive kingdoms, neutralizing them for generations if not longer. Nice job, Ozma!
(And remember what started the whole affair? King Roquat was bored and wanted to get revenge on the Oz people. Who put the idea of an invasion into his head? Maybe . . . someone with a Magic Belt and a Magic Picture?)
Ozma will return in The Patchwork Girl of Oz.