People like cats, yo.
The Doctor was understandably concerned when he found his friends' clothing in piles on the floor. Briefly, he wondered what they could possibly be up to this far into the TARDIS before he noticed what lay near the clothes- an obsolete version of a chameleon arch, one he'd put away in this storage room ages ago. The model had substandard safety features, so it was no stretch of the imagination to assume they'd triggered it somehow. But now the question was why they'd taken their clothes off.
He was debating whether or not he should go find them or wait for them to find him when he heard a tiny mew. There was a small, pale-furred kitten attacking the hem of his trousers.
"Now where did you come from?" he asked, crouching down and picking up the kitten. It gnawed his index finger. The Doctor chuckled, then looked up as he heard loud mewling from the other side of the room. Two more kittens wrestled on the floor.
Three kittens, three piles of clothes, and one defective chameleon arch. "Oh my giddy aunt..." he said as the penny dropped.
He swore once he recalibrated the arch, he'd dismantle it completely and never bother with another one. This particular model had been discontinued once it became apparent that an option to change humanoid beings into non-humanoids had been a terrible idea. It wouldn't be impossible to turn them back, just inconvenient.
First of all, he needed to figure out who was who. The blonde kitten was certainly Polly, but the other two both had tabby patterns. Ultimately, he decided that the one with four white paws was Ben and the other Jamie. With identification out of the way, the Doctor turned his attention back to the machine. Now it was only a matter of figuring out exactly what they'd done so he could reverse it. He began checking the system settings.
A loud crash made him look up. The three little kittens had somehow knocked over a floor lamp and were fleeing from the sudden noise. (Except Jamie, who was walking backwards, arched up and hissing.)
Nobody was hurt, so the Doctor went back to work. Several times, he had to shoo away inquisitive paws, and once he had to stop and remove Jamie from his shoulder. A short while later, he looked up just in time to watch as Polly pounced on Ben from a low shelf. At that same moment, Jamie decided to attack as well, and ended up colliding with her on the way down, knocking a yo-yo across the floor. Ben took off after the trailing string, Polly darted under an armchair, and Jamie fought his own tail for a few moments before running off.
The Doctor chuckled.
He made a few adjustments to the arch and a small panel opened on the side, letting three fob watches fall to the ground. Before he could pick them up, Ben bounded in out of nowhere, knocked two of them across the floor, and chased after one. The Doctor quickly grabbed two of the watches and pulled the third one away from Ben, who was trying to bite through the case. "Enough of that," he chided, turning back to the machine. "Oh, no. Not in there!" Polly had climbed into the side panel and the Doctor shooed her away.
The watches all looked more or less identical, another design flaw of the faulty chameleon arch, but by listening carefully to the mental voices within, the Doctor was able to determine which was which. Carefully, he labeled each one with a felt-tip pen.
Nearly ready, he glanced up to see where the kittens were. Near a stack of clutter, Jamie was wrestling with the end of a feather boa that was dangling out of a box. Attracted to the feathers, the other two soon joined in and gave the boa a big tug. The stack wobbled precariously and-
"Jamie, no!"
-toppled over with an almighty crash. Odds and ends went everywhere and so did the kittens. He found them huddled under the armchair, fur on end and eyes wide. With a little coaxing, he was able to get them to come out. This time, he held onto them while he emptied out a cardboard box and lined it with a blanket and the feather boa. He unhooked three sets of claws from his coat and put the kittens in the box to keep them out of trouble.
Ignoring the mews of protest and the scrabbling at the sides of the box, the Doctor finished resetting the arch, swapped out the old battery for a new one, and cleared a small area for the transformation. It was then that he realised he couldn't hear anything from the box. Quietly, he went over to see.
They'd curled up in the corner and fallen asleep in a pile. Tiny paws twitched. Without waking up, Jamie yawned and rolled over.
The Doctor smiled. Perhaps he'd let them sleep a bit first.
It was about an hour later, and everyone was back to their usual state. In order to deal with the memories from being kittens, they decided to blame the whole incident on alcohol and leave it at that.