The Doctor didn't like mysteries. Well, no, that was a lie. She loved mysteries, just not unsolvable ones. And this current mystery had been unsolvable so far.
It was something that she hadn't realised until she did. Well, that's how realisations worked, right?
They were in the Sahara Desert in 1984. Not intentionally; she was aiming for the Winter Olympics in Sarajevo, because Graham really wanted to see Torvill and Dean perform the Bolero.
So the doors of the TARDIS had opened, and once they realised they were in the middle of a scorching hot desert, everyone shrugged off their winter coats, quickly running to put on shorts and t-shirts. Except Yaz. Well, she had changed into shorts and a vest top, but she still had her reliable leather jacket on. Even the Doctor had got rid of her coat, and she loved a consistent fashion choice.
"You're going to be boiling, Yaz! Here, I'll put your jacket somewhere safe for you."
But Yaz hugged her jacket close to her when the Doctor tried to take it off her.
"No, it's alright, if it gets put down in the TARDIS, I'll never find it again!" Yaz exclaimed, trying to keep the quiver out of her voice.
"Don't be ridiculous!" the Doctor exclaimed, reaching for it again, frowning when Yaz stepped back, shaking her head. "I'll put it by the centre console, and there won't be any risk of it going missing."
"Doctor, I said NO!" Yaz shouted, before seemingly reassessing her tone.
"Sorry," she said softly, "I'm fine for now; I'll take it off later if I get too warm."
The Doctor just frowned slightly, before nodding in response. Two hours later, when sweat was dripping down Yaz's face, the Doctor decided against suggesting her removing her jacket again. She hated arguing with any of her fam, especially Yaz, but decided that this was something that definitely needed investigating.
So, over the next three weeks, the Doctor gradually turned up the temperature in the TARDIS, just a degree at a time. Gradual enough that nobody actually noticed enough to mention it, but enough that eventually everybody was wandering around with bare arms. Except for Yaz, who would not relinquish that damn leather jacket.
Eventually, the Doctor had enough of trying to be subtle in her investigations. So, one evening, she waited until Yaz was ready for bed, and then 'accidentally' barged into Yaz's room, fully prepared to blame the TARDIS for moving rooms around.
Pushing open the door, she was all ready with her excuses, until she saw Yaz sat there, in a tank top and jogging bottoms.
"Woah, that's so cool!" the Doctor exclaimed, causing Yaz to whip her head up in shock, reaching for a blanket to wrap around her.
"W-w-w-what do you mean?" asked Yaz, shakily.
"Reverse tattoos!" the Doctor replied, excitement shining in her eyes. "I didn't think these were due to come in until at least the 22nd century, but you're really ahead of the trend! What are you going for, the spider web? It's pretty abstract, I love it!"
The Doctor watched as Yaz looked down at her arms, before pulling the blanket tighter around her.
"I don't know what you think you saw, Doctor, but I don't have any tattoos."
"Yes, you do! I've just seen them! They're like normal Earth tattoos, but they are raised above the skin and they're always white! The fact that they're above the skin and devoid of colour is why they're called reverse tattoos!"
"No, Doctor, they're not," Yaz protested feebly. "Please, stop asking me about it."
The Doctor was confused about why Yaz would get a tattoo, then want to hide it, but she accepted that she didn't want to talk about it, even if she didn't understand it.
"Okay," she shrugged, before grabbing Yaz's wrist, pulling her out of her bedroom. "Graham! Ryan!"
The boys frowned as they heard the Doctor careening through the TARDIS corridors, shouting their names, eventually appearing in the console room.
"What do you guys know about reverse tattoos?" she asked excitedly.
"Nothing," Ryan said, while Graham just shrugged, raising his eyebrows questioningly.
"Yaz has reverse tattoos, but she keeps hiding them, and I don't know why. She's about a hundred years ahead of the trend, and I was curious how she knew, but she refuses to talk about it. She asked me to stop asking her, so I figured I'd ask you instead!"
"Sorry Doc," Graham said, shrugging his shoulders, "Reverse tattoos just aren't a 'thing' on 21st century Earth."
"Exactly!" exclaimed the Doctor. "I thought they didn't arrive until the 22nd century, but Yaz has them! Look!"
And with that, she pulled the blanket from around Yaz's shoulders with a flourish.
And everyone fell silent.
Mistaking the silence for intrigue, the Doctor carried on talking.
"Isn't it amazing? Where are they doing these in the 21st century? It's so cool!"
"Doc," Graham started hesitantly, "They aren't what you think they are."
Ryan was shaking his head too, and the Doctor was just about to ask for an explanation when she heard a strange noise from behind her. Turning around, she was devastated to see tears pouring down Yaz's face, as she assessed the situation, before turning around and running.
"What happened?" asked the Doctor.
Graham and Ryan looked at each other, before Ryan decided to answer the best he could.
"They weren't tattoos, Doctor. They were scars."
"What? No! How? No! There were loads of them! Far too many to be scars! How would you even get that many scars? Did she get attacked by a bear over twenty times? Because that's the only way I can think she could get so many scars! Although it's odd to only have them on one arm, and her non-dominant one, at that! If you were fighting a bear, surely you would hold up your stronger arm to -"
"Doc," Graham interrupted quietly, "They looked like self harm scars to me, and the one thing Yaz will have wanted, would have been to hide them from us."
"And I just showed you both," the Doctor said, starting to realise that this was something that people didn't like to talk about, judging by the tone of voice being used. "But, what exactly do you mean? Self harm suggests purposely causing an injury to your own body."
When neither Graham nor Ryan reacted, the Doctor stopped her rambling, trying to think through the information that she had just learnt.
"But… why?"
"That's something only Yaz will be able to tell you," Graham explained, "But she may not want to."
The Doctor nodded, considering her options, before darting back into the corridors of the TARDIS, aiming for Yaz's room.