Disclaimer: Doctor Who belongs to the BBC, and Derpy Hooves to Hasbro. The concept for this story, as well as the descriptions of the characters therein, belong to Lissy-Strata.
Of all the confounded things that could happen in this madcap world, this was quite possibly the most irritating the Doctor had yet encountered. It had started when Sgt. Benton escorted a cross-eyed gray pegasus mare into the Doctor's lab to greet him and Jo. She'd had a message from the Master, of all ponies, which immediately set the Doctor on edge. The Doctor had asked the mare's name, and when she'd opened her mouth to respond, all that came out was a loud, forceful hiccup. And that's when the Doctor's perspective of his lab changed drastically. He now found himself no more than ten centimeters tall, with the mare uttering an apology before handing the letter over to Jo and retreating, still hiccuping, as Benton escorted her out of the lab, trying very hard to keep out of her line of fire.
Jo used her magic to levitate the Doctor up to the table top, then opened the letter and began to read. "My dear Doctor, this is my assistant Miss Derpy Hooves. She has a 'little' surprise for you. Yours truly, the Master."
"Oh, of all the childish, confounded pranks!" the Doctor hissed. Oh, how he was going to get the Master for this! "Come, Miss Grant, we must find the Master and demand that he...good heavens, whatever is the matter, my dear?"
The purple unicorn's lower lip was trembling. Suddenly she burst out laughing. "Oh, Doctor! Y-you sound so funny! Your voice...it's as tiny as you!"
The Doctor grimaced at his assistant. "I can't begin to see the humor in this, Jo! The Master is trying to humiliate me and hinder my work, and instead of helping me resolve this issue, you stand there chortling!"
"I-I'm sorry, D-Doctor," Jo managed between fits of laughter. "It's just...this isn't the type of thing we encounter on a daily basis. Usually we deal with things that are a lot more...well, terrifying. Isn't it good to have a case where we can laugh once in a while?"
"This is not a laughing matter!" the tiny Time Pony insisted with a stomp of his hoof.
Jo's only response was to start giggling again.
Just then Brigadier Lethbridge-Stewart walked in. "Doctor, can you explain that peculiar mailmare who...my word, it's happened to you now, too?"
Jo was practically howling by now, which only compounded the Doctor's growing irritation. "It would seem, Brigadier, that that particular mailmare is in the employ of an old enemy of mine; the Master. As far as I can guess, the Master's somehow turned Miss Hooves into a walking tissue compression eliminator, though a far less lethal variety."
A smile crept over the Brigadier's features. Biting back a laugh, he said, "Well, doctor, it would seem that you have a...ah...little problem."
The Doctor rolled his eyes. "Very funny. I'll thank you to refrain from puns regarding my new height and help in restoring me to my normal size! And as an added measure, I would advise you to court marshal a few guards, because clearly they're not doing their jobs!"
"Now now, Doctor, no need to get 'short' with me," the Brigadier said with a barely concealed smile. His voiced squeaked a bit on the word "short," betraying his suppressed urge to laugh uproariously.
The Doctor glowered at the pegasus officer before turning his ire on a still giggling Jo. "Will you desist! This is not funny, Miss Grant!"
"Not even a little?" Jo asked innocently. Then, catching what she said—along with the Doctor's near-wrathful expression—she hastily added, "Sorry. That wasn't intentional."
The Doctor let it pass. "Get my saddlebags, Jo. I'm afraid I'll have to rely on you for transport. Come, we need to track down that mare and find out where the Master is."
Jo complied, lifting the Doctor's saddlebags onto her own back. She then allowed the Doctor to hop into one of the bags. Before giving the command to head out, the Doctor turned to the Brigadier.
"Brigadier, who else did Miss Hooves shrink on her way out?"
"Nopony but you, Doctor. Although she did shrink an important file cabinet, two lamps, a portrait of Prime Minister Heath and our coffee pot. The stallions downstairs are demanding an explanation and a remedy. They've already threatened mutiny if they have to go without coffee for a day."
"Not to worry, Brigadier, we'll find a way to reverse all of this."
The Brigadier nodded, then smirked. "I certainly hope so, Doctor. Otherwise we may have to 'downsize' your department."
"You're a real comedian, aren't you?" the Doctor muttered. "Come, Jo, we've got a renegade Time Pony to find."
"Right," Jo responded, trotting out of the lab.
"Best of luck to you, Doctor, Jo," the Brigadier offered as the two ponies headed out the doors of UNIT headquarters.