A/N: Another fic from AO3 that I never posted here. Just a little Three/Jo fluff, because why not?

Humming to herself as she swept through the UNIT corridors, Jo Grant headed for the Doctor's lab and swung open the door. "Morning, Doctor!"

"Oh, hello, Jo," came the muffled response.

"Doctor?" Jo peered around the room. It looked empty.

"In here, Jo!" the Doctor called. "In the TARDIS!"

Jo stepped tentatively through the open door of the Doctor's blue box. Even after a trip to Peladon in it, she was still slightly wary of the strange machine. She found the Doctor leaning over his console inside, fiddling with the exposed components of an open panel. "Hard at work?" she asked, grinning.

He smiled back. "Yes, actually."

"What are you doing?"

Pulling out what looked like some sort of circuit board, he studied it carefully. "Trying to bypass the main TARDIS drive systems and access the auxiliaries. If I can manage that, then just maybe I can get the old girl operational again..."

"Trying to hot wire it?" Jo giggled. "Naughty!"

"Oh, it's much more complicated than simply 'hot wiring', Jo. I have to deactivate all the main systems, get the backup navigational circuits online, make sure the time rotor is reconnected to the auxiliary drive system, and set most of the controls to manual. A very tricky job."

"So, trying to hot wire it, then?"

He rubbed his neck and returned her grin. "Yes, alright, I suppose so."

"Anything I can do to help?" Jo leaned over the Doctor to peer at his work, unconsciously resting her hand on his side.

"No, I don't think so, Jo, I—" He suddenly jerked away when she touched him. "Sorry, Jo, that tickled."

Jo laughed. "Ticklish, are you, Doctor? Do Time Lords get ticklish? Gosh, I never would've guessed that!"

The Doctor coughed. "You must allow our race some weaknesses, Jo."

"And just how ticklish are you?" Jo put her hands on her hips, eyeing him with a malicious gleam. "Shall I find out? It might come in handy, you know, in case I ever need to overpower the Master."

"Jo... Jo, no! Please!" The Doctor tried to pull away as Jo sprung on him, tickling his sides mercilessly. "Jo, stop this at once! I... I can't... This is too..." His words degenerated into helpless laughter.

Jo didn't stop, resisting his efforts to shove her off. She tickled him through the thick velvet of his coat, beaming gleefully as the usually imperious Doctor was reduced to a half-laughing, half-sobbing mess.

"Oh, Jo, please!" he begged, sliding down to the floor and flopping around like a dying fish. "No more!"

She knelt over him. "Do you surrender?"

"Yes! Yes! You win, Jo! Now, for goodness' sakes, girl, show some compassion!"

"Hmm... no!" She started tickling again.

The Doctor groaned and writhed about, waving his arms in a vain attempt to swat her away. Suddenly there were footsteps outside.

"Doctor? Doctor, where are—" The Brigadier entered the TARDIS. "Oh, there you are, Doctor, I— Good lord, what's going on here? Is he having some sort of fit?"

Jo jumped away from the Doctor, her face turning red, and the Doctor stopped moving, letting out a relieved sigh. "Thank heavens, Brigadier," he murmured. "I've never been more grateful to see you in my life."

The Brigadier stared at them both, his eyebrows raised and a perplexed frown fixed beneath his moustache. "Look, would someone please tell me what's going on here?"

"You know," said Jo, helping the Doctor to his feet, "I've discovered something today, Brigadier. Venusian aikido's all well and good, but if you really want to get tough with someone, nothing beats good old-fashioned tickling!"

"...Tickling?" The Brigadier looked at Jo, then to the Doctor, then back at Jo. "Sometimes I think I'm better off not knowing just what goes on around here."