Written for the "communications" challenge at the fan_flashworks livejournal community. Set between Winter Holiday and Pigeon Post. Since Titty Walker's name isn't given in canon, I named her Mavis after Mavis "Titty" Altounyan, her real-life inspiration.
"Can you believe it?" Nancy complained. "She's going to read every single one of our letters! And I can't even call Titty Titty, or sign as Nancy, it's got to be to Mavis from Ruth."
"Awful luck getting landed with Miss Pearson when Miss Alden had to leave," Peggy sympathised. "Miss Alden wasn't nearly so fussy about letters."
"I know, especially when they're to people we've been writing to for ages, like the Swallows. It was bad enough not being allowed to write to John and having to wait until Susan or Titty could write to him and get an answer, but now it's going to be really hard to plan anything for next summer."
Peggy nodded. "I'd write if I could, but you know how strict Miss Dawes has always been about checking letters. Even you couldn't get away with much when you were in her form!"
Nancy sighed and sat down next to her sister. It was a lovely afternoon, and normally she'd have been enjoying the rare luck of sunshine on a Saturday, but today she was too annoyed.
"We need to find a way to get messages through secretly. Maybe I could do a semaphore picture like the one I sent you about the Fram?" Nancy suggested.
"Umm. You'll get into an awful row if Miss Pearson knows semaphore, and she'll be watching you for the rest of the term."
"Not many people know semaphore. Although I think she did mention something about the Girl Guides once. They learn semaphore, don't they?"
"Yes, I think so... maybe we could hide the message somehow? Sew something and put the message inside?"
"Sewing's bad enough for class, I'm not making anything more than I have to! But hiding the message is a good idea..."
They were both silent for a moment, thinking.
"Jib-booms and bobstays, I've got it!" Nancy said, careful to keep her voice low. Her eyes, though, were dancing with excitement. "Remember when we were telling Uncle Jim about how we'd been sending secret messages, the day after we went to the Pole? He said he'd have used milk or vinegar for invisible ink, then realised that since the doctor was heating everything before handing it over it wouldn't have worked. But these letters don't have to be heated, so it should be fine."
Peggy wasn't convinced. "We can get some vinegar or milk from the kitchen easily enough, but will Susan or Titty realise they've got to heat it?"
"I'll put something about milk and heat in the letter, and I'm sure they'll figure it out."
"Better write to Titty. She's the one who realised how to read your semaphore message, and she likes writing more than Susan," Peggy suggested.
"Yes, and if I'm calling her Mavis, she must realise there's a secret message somewhere. Come on, let's go and write the letter!" Nancy leapt up and set off.
Miss Pearson looked suspiciously at the letter Ruth Blackett had just handed her. It seemed to be a perfectly ordinary letter, from one schoolgirl to another, but even a short acquaintance with the elder Blackett girl had made her rather wary. She couldn't, however, find anything wrong with it, so she handed it back reluctantly.
"Very well, Ruth. Seal the envelope and give it to me to post."
"Yes, Miss Pearson," the girl replied demurely. Only when the teacher had moved on to another classmate, did she allow herself a tiny smile. Miss Pearson might not know it, but the Amazon pirates had won.
Transcript of the Letter (secret message in capitals).
POST CENSORED. SECRECY VITAL.
How are you? I hope you are well. I am very well, and happy to be back at school. I have a new Form Mistress, Miss Pearson. She reminds me of my Aunt Maria. I'm sure you remember my aunt.
MUCH TO PLAN. FURTHEST SOUTH? WAR ON C. FLINT? BETWEEN US?
Do you think your family will visit the Lake again this summer? I hope you do; we could have great fun. Please give them my love next time you write. Margaret sends hers, too.
THERE'S THE DS TOO. WHAT ARE THE SWALLOWS' THOUGHTS?
Do you remember being in Uncle Jim's houseboat last winter, when it was so cold that the milk had frozen? We won't have that problem in summer. The heat will be the problem then.
WELL DONE AB ON CRACKING THE CODE!
I look forward to hearing from you.
SWALLOWS, AMAZONS AND DS FOREVER!
You can see photos of the actual letter I created (before and after heating) in this webpage (remove the spaces): fan-flashworks . livejournal/ 33969 . html
While we never see Nancy's handwriting, her printing is in a couple of illustrations, and is much neater than mine. My excuse is that she was in a hurry, and it's hard to be neat when you can't see what you're writing!