"You're going to land the TARDIS in my mind?" Luna asked, looking truly surprised for the first time today.
"Going to try to," the Doctor panted back as he literally sprinted about the controls.
"Will I still get to walk around and things?"
"Luna Lovegood, you sound like Martha."
"Martha who?"
"Oh, now you do the thing." He yanked at a lever.
"How does that work? The mind thing?"
The Doctor paused just long enough to answer, "A mind is like a TARDIS: bigger on the inside and many places at once. But the most important thing for you to remember, Lovegood, is that it's your mind. The Great Intelligence can't put anything there that isn't there already, but once it's there, it's a weapon that he can use against you."
Luna blanched but nodded pensively. "It works the other way, too, though, doesn't it?"
"Exactly!" the Doctor agreed brightly, and he ruffled her hair once more before returning to his sprinting and lever-pulling. "Now, we need a landing strip here...Can you think up maybe a really sunny meadow?"
Luna conjured up all of her focus. Blue sky, loads of grass and flowers...bunnies...
The TARDIS slammed into the ground, jarring her.
"Oh dear." The Doctor scratched at his prominent chin. "Sorry about that. Now, let's go, Ravenclaw!"
"What?"
"What? Come on, get a move on!"
Luna clambered to her feet and met the Doctor at the door.
"Think meadowy thoughts," the time lord reminded her before opening the door...
...to a beautiful, green meadow.
Luna sighed in relief and followed the Doctor out into the sun and the blue.
Then she saw the woman. She was tall, with long, brown hair, and clad in a bright blue sundress. She danced about and laughed as though the wind were whispering wonderful things in her ears.
Luna inhaled. "Mum?" she called out, but the familiar woman didn't hear her.
At the same time, however, the Doctor, in a perplexed voice, called out, "Clara!"
Luna shrieked louder, "Mummy!"
The woman at last turned to look at her daughter, only to vanish into thin air immediately after.
"No!" Luna shouted, then concentrated on conjuring her mother up again. No matter how hard she focused, nobody appeared. She felt tears sting her eyes, which was new for her; she didn't normally cry. That was her dad's job. Her job was to stay happy.
With this in mind, she shuddered and wiped the tears away. She could feel herself becoming angry, but she suppressed it.
Suddenly, however, the ground where her mother had been standing split open, sending a crack travelling through the rolling hills and separating Luna and the Doctor.
"Lovegood!" the Doctor exclaimed. "Calm down!"
"I am!" she replied as the chasm betwixt them grew.
"Only on the outside," the Doctor called. "We're on the inside."
Luna nodded slowly. "That makes sense." She felt another tear run down her cheek, and she allowed it, focusing instead on breathing, making peace inside of her.
The ground stopped splitting, but the Doctor was out of sight. Before her was a vast sea. She pursed her lips and shouted out, "This is my mind!", and her voice echoed and reverberated throughout all corners of the world on which she now stood. "You can't win inside my own mind!"
That was when she experienced the familiar breathless sensation and passed out again.
...
When the ground stopped moving, the Doctor found himself in an odd sort of marsh...Well, "odd" was a relative term. It had all of the murkiness characteristic of your run-of-the-mill marsh, but it was filled with whispering voices and shimmering faces that appeared and disappeared in the blink of an eye, like wil-o-the-whisps.
The Doctor straightened his bow tie and was about to enter the marsh when a familiar voice spoke behind him: "They're wrackspurts."
Turning around, the Doctor saw Clara Oswald, only dressed in a blue gown and drifting like a ghost or a siren. "What?" he said.
"The voices," Clara answered. "They're wrackspurts. The glowing things are nargles."
"Clara?" the Doctor murmured. Alright, so there's another version of Clara floating about who is Luna Lovegood's mother, he thought. So what? That's how time streams work. "What are wrackspurts and nargles?"
"When the wrackspurts get louder, you'll know you're close," Clara said, her form flickering, "but don't listen to them, or you'll lose your mind."
"Funny thing, a mind," the Doctor rambled. "Always a thing you find when you're not looking for it."
"Clever boy," Clara sighed, then vanished.
"Oh, come on!" the Doctor called out plaintively. Then he squared his shoulders, shook out his arms, and started into the marsh.
...
Luna woke with a start and found herself bound in a room that resembled her own, only gigantic and dark. She stood, faintly aware of the cold against her bare feet, and stuggled physically and mentally against the ropes that bound her wrists. This is my mind, and in my mind, I choose NO ROPES.
It seemed that the darkness was laughing at her. The voice was very deep and eerie. "Of course I can not kill you," it mused, "but I can keep you trapped here, in the midst of your greatest fears, tucked away within the shadowy regions of your mind."
Luna took a deep breath and thought that over. "Honestly, trapped is trapped, isn't it? Why go to the trouble of keeping me in a nightmare when you could just let me stumble about aimlessly? You don't need me to suffer."
The reply was brief and malevolent: "I get bored."
Then a chill fell over Luna, and something crept out of the shadows.
It was a human, and, despite the dim lighting, it seemed familiar.
"Dad?" Luna said cautiously.
Then the figure fell to its knees and started convulsing, morphing, growing. Horns sprouted from its head, and its back arched, and fangs grew from its face.
Luna didn't hesitate to run, but it was like dream-running; her position in relation to everything else in the room didn't change. Not really.
So instead, when the Crumple-Horned Snorcack bounded toward her, she sat down cross-legged, closed her eyes, and dreamed of sunlight.