S omeone," Marcia told Catchpole, "has defaced my door."

Catchpole jumped up guiltily, his sparse sandy hair standing up in surprise. Marcia had caught him taking a quick nap in the Old Spells cupboard. "Oh," he said.

"If this is your idea of a joke I don't think it is very funny," said Marcia icily.

Catchpole balanced on one leg like an embarrassed heron. He wasn't sure what Marcia was talking about but it sounded like trouble - again. "Oh, dear," he said.

"Well, is it?"

"Is it what?"

"Is it your idea of a joke? I know your penchant for drawing on doors."

The penny dropped. "Oh, no. It wasn't me, I promise. Absolutely not. Honestly - it wasn't."

Marcia sighed. She believed him. The bizarre scribbles were far too complicated for Catchpole to have done. "Well, go get a bucket and a scrubbing brush. I want them cleaned off. I'm off to see Sarah Heap and I expect a nice clean door by the time I return. Got that?"

"Got that, Madam Marcia. Will do." Reprieved, Catchpole shot off to find a bucket and a scrubbing brush.

"No!" Jenna gasped. "It's disappearing! Stop. Stop!" In front of them the map was vanishing.

"Quick, tell it to stop," said Nicko.

"Stop!" yelled Jenna.

"No - no, I mean write on it. Quick, Jen, before it all goes."

Jenna picked up the piece of chalk and scrawled: STOP! DO NOT ERASE.

Catchpole screamed and dropped the bucket of hot soapy water on his foot. Huge, looping letters were writing themselves across the door as he watched. It was worse than when he had started - what would Marcia say? Catchpole picked up the scrubbing brush and got to work with a vengeance, but even as he scrubbed, more words appeared in the very spot he had just cleaned. Suddenly Catchpole understood - this was a test. Marcia had set it so that he could prove himself worthy of being reinstated as a sub-Wizard. Catchpole was determined not to fail. As more and more words came into view telling him STOP! THIS IS AN URGENT MESSAGE! Catchpole sped up, catching each one with his scrubbing brush as soon as it appeared, splashing water everywhere. Soon the landing outside Marcia's rooms was a large, chalky puddle.

"More chalk!" yelled Jenna. "Quick!"

Snorri handed her a stub of chalk. "It's the last one," she said.

Jenna stopped, her hand poised above the door. She could not risk wasting this precious last piece of chalk. They watched MARCIA, WE ARE HERE! disappear from the door, followed by the rest of the precious map until nothing remained of Jenna's messages. "It's not going to work," she said miserably. "The door just gets rid of it."

Everyone fell silent, a feeling of despair hanging in the air. Suddenly Septimus said, "It did work. But someone is washing it off."

"Who would do that?" asked Nicko.

"Marcia wouldn't," said Jenna, "or any of the Wizards. They'd know it was important."

"So who would be so stupid?" said Nicko.

Septimus knew exactly who. "Catchpole," he said.

"Catchpole?"

"Yep. It has to be. No one else in the Tower would dream of doing that. Jen, give me the chalk. I know what to write."

Jenna handed over the chalk. She hoped Septimus knew what he was doing.

IS THAT YOU, CATCHPOLE? Septimus wrote in very clear letters.

"Is that you" was quickly erased, but the rubbing out stopped at the "C" of "Catchpole."

"I'll wait for him to reply," said Septimus. "There's no point wasting any more chalk until we know he's figured it out."

Outside the Twin of Marcia's door five people watched with bated breath. Seven long minutes passed while Catchpole threw the spiral stairs into fast mode and zoomed down to the Old Spells cupboard to get his pen.

He returned to find an irate Marcia accompanied by an anxious Sarah Heap - who Marcia had bumped into under the Great Arch. Marcia was staring at the door, her robes gathered around her ankles, her purple pythons soaking up the chalky water like a couple of pointy sponges. Catchpole jumped off the stairs, skidded across the soapy floor and careened into his bucket, sending the rest of the water flying over Marcia. "What do you think you are doing?" she exploded. "I ask you to perform the simple task of removing graffiti from my door and you have the cheek to daub it with your own name. Catchpole, this is the last straw. You are fired!"

Sarah Heap looked shocked. No wonder Septimus had run away if Marcia spent so much time yelling like this.

Catchpole was horrified. "No!" he pleaded. "No, it's not what it looks like."

"Ha!" said Marcia. "I've heard that one before. Believe me, Catchpole, it generally is exactly what it looks like - and then some."

Catchpole produced his pen and waved it desperately. "But I was just - "

"I have no need to see what you've been writing with, thank you," said Marcia. "I have better things to do. Stand aside, will you?"

"No! No, you don't understand." Catchpole threw himself in front of the door to stop Marcia from going inside. "Please, Madam Marcia, please. I didn't do it. I can prove it. Please." There was a break in Catchpole's voice that caught Marcia by surprise.

"Very well," she said. "Prove it."

"Oh, thank you, thank you, thank you!"

"For heaven's sake, stop groveling. Just get on with it."

Oblivious to the soapy water, Catchpole kneeled down and wrote on the door, IT IS I, BORIS

CATCHPOLE. WHO ARE YOU?

Marcia tapped her foot impatiently, making little splashing noises. But as the words SEPTIMUS

(BOY 412) appeared, the splashing noises stopped. Sarah Heap screamed.

"See?" said Catchpole. "It does it on its own. It's said lots of things."

"Like what?" asked Marcia.

"I don't know," Catchpole replied. "I was too busy washing them off."

"You idiot! You washed them off?"

"But you told me to."

"Oh, for goodness sake, give me your pen." Marcia snatched the pen from Catchpole's trembling hand and wrote: SEPTIMUS, IT'S MARCIA HERE. WHERE ARE YOU?

Far away in the House of Foryx a loud cheer went up.