Author's Note: I don't have a lot to add to this note, but I'll have a lot of story notes at the bottom. So enjoy, and allons-y!

Chapter One- Close Encounters

"Never get tired of hearing that," The Doctor said, grinning. "No, it's just 'the Doctor.'"

"You're not the Doctor," Batman said. "I've met him."

"Ah, yes, well, actually that was me. Or at least, it used to be me. I was him, but now I'm me."

"What?"

"You met me in a previous life, oh, must be… six lifetimes ago actually. Remember, the Cyberman invasion? That was a fun one. Cybermen in Gotham, kidnapping city officials, trying to convert the Joker, great fun, that."

Batman seemed to relax slightly, but still kept the Batarang in his hand.

"How do you know of the House of El?" Superman asked.

"Well you are wearing their crest on your chest. Red Blue and yellow- bold fashion choice. Of course, this from the man who once thought wearing a vegetable was the height of fashion."

"You know Kryptonian symbols?" Superman asked, ignoring the Doctor's comments about his wardrobe.

"Yes. Well, actually no, not really, but the TARDIS translates automatically for me."

"The what?"

"It's his ship," Batman answered for him.

"But I have been to Krypton, yes, a few weeks ago, in fact" The Doctor added.

"Not possible- Krypton has been gone for decades!"

"Yes, but I have a time machine. So let's see- wearing the crest of the House of El, clearly acting as a hero on Earth, that must make you… Jor-El's boy, Kal-El, am I right?"

"How did you…"

"I knew your father. Itried to help him convince your people of the tragedy that was approaching, for all the good it did. I was thrown off the planet for my trouble. A great man, your father was. Wise beyond his years and intelligent beyond his own planet's obstinacy. And yet, when I offered to take him and his wife from the planet, he said no. He insisted he could still convince them, despite Braniac's denials of danger. Mad, that computer was, completely bonkers."

"If you have a time machine, couldn't you have saved Krypton?" Superman demanded.

The Doctor's expression took on a darker tone, and he frowned. "No, I couldn't. Krypton's destruction is a fixed point in time. I'm sorry. I'm so sorry."

There was a silence, but Batman finally broke it.

"What are you doing here, and how did you get in here without us hearing you or your TARDIS?"

"Well, like I said, I was chasing down a rogue member of the Slitheen, and the TARDIS took a wrong turn. Wound up in a closet on the next deck down. I don't make a lot of noise when I walk, wearing trainers, you know. Or 'sneakers' as you Americans call them for some reason, well; I guess I know now- you can 'sneak' up on someone with them. You know, I…"

"What's a Slitheen?" Batman asked, interrupting the Doctor's rambling.

"Not a what, a who," the Doctor replied. "They're a crime family from the planet Raxacoricafallapatorius. They like to take human skins and stuff themselves in them. Nasty ones, those. Nearly caused World War three a few years ago when they faked an alien invasion, just so they could sell off the irradiated remains of Earth as fuel."

"And there's one of them here, on Earth?"

"Yes. He escaped from an Atraxi prison and headed straight here. He wants revenge for the destruction of her family members. And he'll do anything he can to get it."

888888888888888888888888888888

A pond 15 miles from Syracuse, New York

Jackson Thorn sighed in frustration as he cut the fishing line to his best rod, which had become snagged on a tree hanging out over the water, too far for him to reach without a boat. He had been out all die, but hadn't caught anything but the tree, and on top of that he was developing a nasty sunburn. A former soldier, he had to admit that he had let himself go lately, and had gained quite a few pounds since his discharge from the army. With all the stress he'd had lately at work and at home, he was hoping to have had a relaxing, stress free day today, heaven knew he needed it! But no, that just didn't seem possible at this point.

After reeling the line in, He was about ready to give up and go home when, with no warning, he was knocked off his feet by a shockwave as something struck the water.

Soaking wet, he sat up as the air cleared around him, and was surprised to see nothing more than a crater in front of him where the waterline had been before. No, it was more than just a crater, there was something in it- a silver craft, with the appearance of something straight out of a 1950s sci-fi movie! There were reentry burns around the hull, and something…more. It almost looked like battle scars around what appeared to be the ship's engine.

Jackson swallowed the lump in his throat, and went to his truck, keeping an eye on the crater. He pulled a rifle from the rack, and loaded it. He then lifted it to a ready position, and approached the craft.

"Hello?" he said. "Is someone there? Yer not one of them crazy Hawkmen, are ye?"

As there was no response, he went closer to the craft, reaching his hand out slowly. Just before he could touch the ship's silver hull, there was a hiss, and a crack appeared in the front, smoke rolling out of it. Jackson jerked his hand back and once again raised his rifle.

"Alright, I don't want no trouble, jest come out of there slowly with yer hands up!" He demanded. The only reply was the hissing of the escaping smoke.

Maybe it's just an empty shell, he thought to himself. Unfortunately, as he was watching the hatch open in front of him, he didn't see the other hatch in the opening behind him, or the green clawed hand that emerged from it. He didn't see the creature emerge from the ship either. But then he heard something behind him as the steam stopped hissing, so he turned and desperately fired the rifle, seeming to miss the creature that had suddenly appeared before him.

"That was a mistake," the creature said. "You're a bit small, but you'll be adequate for a disguise."

The last thing Jackson Lake saw were the cold black eyes of the creature as he breathed his last breath.

88888888888888888888888888888888888888888888888888 888888888

The Watchtower Conference Room

"The Green Lantern corps is familiar with Raxacoricafallapatorius, it's in the next sector over, but I've never heard of the Slitheen. I take it they could be a threat?" Green Lantern John Stewart asked.

"What? Oh, yes," the Doctor replied after a moment, staring at J'onn J'onnz, better known as the Martian Manhunter with a grin. "I'm sorry, I just can't get over the fact that a green Martian is here, right in front of me! Even the Ice Warriors thought you were all wiped out by the white Martians!"

"The who?" Flash said.

"The Ice Warriors- humanoid lizards, basically, a noble warrior race, native to Mars but long since moved to other planets."

"We need to focus," Batman said, bringing them back on task.

"Sorry, you're right, there's no telling how long the Slitheen's been on Earth. The TARDIS is funny that way, traveling through time and space, but sometimes ending up either too early or too late."

"Can't you just go back and try again?" Wonder Woman asked. The Doctor shook his head.

"Doesn't work that way, I can't go back on my own timeline. Doing so would have catastrophic consequences."

"Have I mentioned how much I hate time travel?" Green Lantern John Stewart added, remembering the League's experiences in Vandal Savage's altered timeline.

"Not really, no," The Doctor said. "What experience do you have?"

"That's not important right now," Batman cut in. "Doctor, do you have a way to track this Slitheen?"

"Nope. Well…yes, but no."

"What do you mean?"

"I can track the Slitheen's ship from the TARDIS, but not the Slitheen himself. Now that I'm closer, the TARDIS should be able to lock onto the ship and take me directly there."

"Then we're coming with you," Superman stated matter of factly.

"Don't think that's a good idea…"

"We're coming." Batman said.

"Well okay then," The Doctor said, standing. "To the closet then- allons-y!"

"J'onn, Diana, you stay here in case of emergency," Superman ordered.

"What do you mean closet?" Flash asked as they walked down the hall.

"Oh, that's where I parked the TARDIS."

"You parked a spaceship…in a closet?"

"Well, it's not exactly a spaceship, as such," the Doctor replied as the door opened to reveal a blue police box from the 1950s.

"How on Earth are we going to fit in that?" Green Lantern asked.

"Oh, you'll see," the Doctor replied as he opened the door.

The group gazed in wonder at the console room inside. Flash ran in and around it at top speed, then came back to the group at the door.

"Butt.. It's bigger on the inside!" he stuttered.

"Never get tired of hearing that either," The Doctor said with a grin. "That's Time Lord Technology! Time And Relative Dimension In Space! Now if you'll all kindly step inside, we'll get underway!"

Outside the box, J'onn raised an eyebrow as he watched the box slowly fade out of existence with a wheeze.

"Well that's…interesting," Wonder Woman said.

88888888888888888888888888888888888888888888888888 88888888888888

Author's Note: So I took a few liberties to put these two universes together. One of the biggest liberties is that the JL universe and the Doctor's are the same, but the JL time period is a decade or two ahead of companions Clara, Amy, Donna, and the others we've seen so far. In the Doctor's personal timeline, it's just before the Waters of Mars, shortly after his foray on the desert planet with the bus and the thief (I can't remember the episode title). And before anybody asks, no, I haven't actually written a crossover between the Fourth Doctor and Batman involving the Cybermen and the Joker, so if anyone wants to take a stab at it, feel free.

I tried to work the Green and White Martians into the Doctor's Mars (with the Ice Warriors and water creatures) by making them older than the Ice Martians, or at least contemporaries. I'd enjoy any comments on the plausibility of that. As far as the Green Lantern Corps knowing about Raxacoricafallapatorius, I figured it would make sense in context. I think of it like maybe the Shadow Proclamation is either an extension of the corps or a separate entity, perhaps like the UN or the local government, where the Judoon are a hired security force while the Lanterns are a separate entity that still follow and enforce local laws, or something like that. I'll add more details as needed, but it's basically a minor plot point, so I doubt I'll need to expand on it.

GL's references to time travel refer to the JL three part episode "The Savage Time."

Anyway, that's about all I can think of on the story. If you have any questions, PM me, and as always, I welcome any comments, suggestions, and/or criticisms. Thanks for reading!