The Pan-Dimensional Pentagram
Donna was the one who spotted it. They were in a shop on some far-off planet—the Doctor knew the name, but she couldn't even pronounce it—when something on the back wall caught her eye.
It was a medallion of some sort, about the size of a dinner plate. It looked to be made of bronze, and had a large five-pointed star across the front. There was a hole in the top with a thin silk cord strung through it. The cord was long enough that it could, theoretically, be worn as a necklace. Though I dunno who'd want to, Donna mused. Put that thing around your neck, you couldn't even stand properly.
"You like?" asked the owner, jolting Donna out of her thoughts. "Very pretty medallion. Everyone who come in look at it, but no one buy. You make me an offer, I sell to you."
"Sorry, we're just looking," Donna told him. "I haven't even got any money…just these coins, see?" She held out the change she'd found in her pocket.
The shop owner's eyes lit up. "But this is Earth money!" he cried. "Very rare out here, very rare indeed. You like to buy it? We make a deal."
"Really? Well, all right then," she agreed, surprised. She wasn't even sure what to do with it. There was just something about it…some sort of force that drew her to the medallion like a magnet…
"What'd you find?" asked the Doctor.
"Some sort of medallion…oh, it's quite heavy…it's nice, though, don't you think?" She held it up.
The Doctor's eyes grew wide. "Let me see that!" he cried. She handed it over. He looked at it closely all around the edge. He turned it over; Donna could see an identical star on the back. "Is that what I think it is?" he cried excitedly. "It must be! But I thought that was only legend…oh, this is bloody brilliant!" He scanned it with the sonic screwdriver, and Donna was sure the medallion lit up somehow.
"That's weird," she commented.
"Donna, do you know what this is?" he asked. "It's the Pan-Dimensional Pentagram!"
She stared at him, unimpressed. "And that's what, exactly?"
He turned around and ran towards the door. "Back to the TARDIS! Come on!"
Always with the running, she grumbled to herself. "But what is the panda-whatever-it-is? Doctor, is it dangerous?" She didn't think he'd be running towards his ship if it was, but with the Doctor, you never knew.
"In the wrong hands, it could cause untold destruction!" he called back over his shoulder. "It's very powerful. But for us, it opens up infinite possibilities. Just think of it, Donna!"
"You can already travel anywhere in time and space," she pointed out. "What does this thing do?"
They had reached the ship. The Doctor opened the door and they went inside. He took the medallion over to the control panel and pressed a few buttons. Something popped up from the dashboard.
"Is that a hat rack?" asked Donna.
He placed the medallion on the end of it so that it was hanging down near the ship's core. "Theoretically, the Pan-Dimensional Pentagram should catalyze the positronic ions in time particles, causing the surrounding electrons to shift in their sub-dimensional fields…" He drifted off, noticing that Donna was giving him a blank stare. "Long story short: inter-dimensional travel. Alternate realities."
"You told me that was impossible," she pointed out.
"It is! Of course it is! But this will allow us to travel to realities that are so far from ours, we never even could have existed! From where we're sitting right now, these worlds—these planets—don't even exist. But they could have! And now we can travel there! We could visit Earth as it never was. Who knows what we'll find?" He began to start up the ship, turning dials and shifting levers faster than Donna had ever seen before. The medallion began glowing again with a bright purple light. "Right, then! Off we go!"
The TARDIS gave a violent jerk. Donna struggled to keep her balance, and just managed it by grabbing on to the control panel. "Doctor!" she cried over the din of the TARDIS's start-up sound, "Are you sure this is a good idea?"
"No!" he shouted back. "But we'll never know until we try, right?" The medallion began emitting a loud, high-pitched whistle. It spun around, slowly at first, then faster and faster. "Marvelous! It's working!" the Doctor cried over the noise. Donna considered covering her ears, but that would mean letting go. The TARDIS shook again.
A few moments later, the noise mercifully stopped. Unfortunately, the shaking did not. "Isn't this exciting?" the Doctor cried. "I can't believe you found the Pan-Dimensional Pentagram just hanging in a shop! How much did you pay for it?"
"More than I should have," she muttered. "Far, far too much."
Author's note: When I first started watching Doctor Who I immediately thought of the possibilities for crossovers. Most of my ideas right now are for American shows (I'm American) but if you have a crossover you'd like to see, let me know! Also, their adventures with the Pan-Dimensional Pentagram are going to be considerably less serious in tone than your average Who episode. This is just for fun.