Hey there! This is my first attempt at a crossover story. As such, I leave all readers fair warning: if it doesn't work out, I will drop it and delete it (unless someone else wants to pick it up...).

Now, as a crossover between the Phineas and Ferb show and the Halo video games, this will have quite a bit.

Halo fans, be warned of jokes you will only get if you watch the show. And, I know this story will be completely disproved in Halo 4 (even if this could happen).

Phineas and Ferb fans, be prepared for specifications on weapons and other rote facts that only Halo fans will know.

This is rated T due to: violence, blood, gore, probable swearing (may or may not happen), and disturbing images. Also contains: romance, impossibilities accomplished, humor, semi-aquatic mammals, adventure, British accents, family themes, and giant floating baby heads.

Rating may be subject to change. If anyone thinks it should be upped to M, please let me know.

YOU HAVE BEEN WARNED!

Disclaimer: I don't own Phineas and Ferb. I don't own the Halo franchise, even if I own most of the games (on disc).

Chapter 1: Of Wormholes and Reentries

Date: November 7, 2554

Location: UNSC Forward Unto Dawn

Personal: UNSC Artificial intelligence serial number CTN 0452-9 and Master Chief Spartan 117

Status: Unknown

Cortana gave a mental sigh. Somehow, she had thought that they would have been rescued by now.

This, of course, was pure speculation, and had no real basis in fact. But after she had been paired with John and had time and time again witnessed his unusual amount of luck, she had begun to expect it at almost every turn.

But after a couple years of drifting in space, she had to face it: their luck had run out. It was amazing that they were alive at all, even if that wouldn't last. While John could theoretically last forever in cryo, he would eventually be left alone. She only had two more years until her thought systems overrode her basic functions and she would unravel.

She wasn't afraid to die. She had faced the prospect too many times and come too close to be afraid.

She was afraid for John.

The thought had, at first, seemed ludicrous. She was a computer program, regardless of how intelligent, and he was a Spartan, the greatest soldier ever created. He had been the key to humanity's survival. He didn't need anyone.

But here, now, he was helpless. Asleep in cryo until someone woke him up. If- When she died, she couldn't be certain that the next being to come along wouldn't kill him while he slept.

If anyone came at all.

Her avatar flickered to life in front of the cryotube. At times, she wished she could wake him, just to be able to talk with someone. But, his last words had been clear: Wake me when you need me. She didn't need to talk to anyone. She would wait until-

A sudden tremor startled her. Speeding through the remains of the ship's computer systems, she accessed the exterior cameras.

Only one was functioning. But one was enough.

What looked like a hole in space had opened in front of the Forward Unto Dawn. And the ship was drifting toward it.

A quick diagnostic showed that there was no way to avoid it now. There was nothing she could do.

I guess this might qualify.

She returned to the cryobay computer and started the wake-up process.

Date: First Age of Truth

Location: Carrier Shadow of Intent

"So, Admiral, you still doubt the importance of this?"

Fleet Admiral Terrence Hood shook his head. "No, and I am glad the first ship available was the Legacy."

Thel 'Vadam let out a low chuckle. While the human ship, Legacy of War, was impressive for a human ship, it would still be no match for most ships under his command.

Yet, it might come in handy. Especially since no one knew what was on the other side of this… wormhole.

It had appeared in orbit over the remains of the planet Joyous Exaltation. It was almost as if someone had taken the fabric of space and torn it open. It was not at all like an entrance to the void, but clearly something different.

"So, Arbiter, what do you propose we do about it?"

'Vadam returned his gaze to the communications screen. "It is simple. We go through it."

Date: Fortieth Age of Conflict

Location: Supercruiser Blade of Judgment

The Prophet of Judgment examined the anomaly before his vessels.

It was not an entrance to the void; that much was plain. If not made by mortals, it must have been a gift from the Gods. Perhaps, it was a path to undo what the treachery of the Sangheili had done.

"Shipmaster," the self-proclaimed Hierarch commanded, "take us through the portal. It is the will of the Gods."

"Yes, Hierarch." Brutarus bowed before turning to a lower pack member. "You heard the Hierarch. We're going through."

In an alternate universe…

Perry burst through the door, only to be trapped in a model skyscraper.

"Ah, Perry the Platypus, how astonishing. And by astonishing, I mean COMPLETELY TONISHING!"

Perry rolled his eyes as his nemesis, Heinz Doofenshmirtz, managed to make himself look like a fool. Again.

"Well, I bet you're wondering what my plan for today is, hm? Well, I was looking for a place to take a vacation last week and I decided to go to a place with a big tourist attraction. I eventually decided to visit the largest manmade building in the world!" He flung out his hands with his last sentence, attempting to appear dramatic. Then the evil scientist scowled. "Unfortunately, everything there was way outside my budget. That's when I got this idea! If I eliminate every building taller than mine, it'll become one of the biggest tourist attractions in the world!"

As Doofenshmirtz cackled madly, Perry began to chip away at his plastic prison.

His cackling faded. "Of course, there is a side-effect. The longer it fires, the more likely some sort of space-time hole will open in orbit around Earth. But I did the math, and the couple seconds it'll take to shot around the world won't open one. You know, probably. Anyway," he turned to his machine, "time to activate the Building-Annihilate-Inator!"

Perry pushed at a fracture he had made in the model, which caused it to shatter.

Doofenshmirtz pushed the fire button.

Perry leapt and kicked him in the face.

"Perry the Platypus! Had did you escape?"

Perry didn't respond, except to begin attacking Doofenshmirtz.

Behind them, the Building-Annihilate-Inator continued to fire a single beam toward an escape tower controlled by a very rude computer, which currently encompassed the entire galaxy in a dome. Which, as it had been made in the same town that Doofenshmirtz resided in, made it the largest manmade structure created.

A few minutes later…

As Perry prepared to finish the fight, Doofenshmirtz suddenly held up his hands. "Perry the Platypus, do you hear that?"

The pair turned to see the –Inator still firing. Eyes wide, they both rushed toward it.

Doofenshmirtz reached for the cancel button when the beam finally stopped and turned toward another target. He looked at the time that beam had fired for and gasped.

Perry pushed the self-destruct button and the –Inator vanished in a flash of green light. He waited for the usual "Curse you, Perry the Platypus!"

It never came. Perry looked at Heinz and saw him writing a long math equation on a whiteboard. Once finished, the evil scientist gasped again.

"Perry the Platypus! That beam fired so long that there will be three space-time holes! THREE!"

Perry face-palmed. Leave it to Doof to screw everything up.

Date: November 7, 2554

Location: UNSC Forward Unto Dawn

Personal: UNSC Artificial intelligence serial number CTN 0452-9 and Master Chief Spartan 117

Status: Operational

"Chief, can you hear me?"

John, also known as Spartan 117 and the Master Chief, opened his eyes. He immediately noticed that the cryotube was already open.

"Thank goodness. You weren't responding."

John looked at the pedestal. He nodded at Cortana and asked, "How long?"

She grinned. "Just under two years. But we do have a problem."

"Well?"

"We're about to go through some sort of wormhole. I have no idea what is going to be on the other side. Plus," she rubbed the back of her head sheepishly, "your armor is better shielded than the remaining systems of the Dawn. I'm not sure that they'll survive the trip through."

John grinned. She didn't have to say she was scared for him to know that. He knew her too well.

He reached down for the chip, ejecting it after her avatar vanished. He placed it in his Neural Interface. He felt the familiar chill of her mind entering.

"It's good to be home."

John chuckled. "What, starships not good enough for you?"

"Between you and this scrapheap, I'd choose the one with locomotion, thank you very much."

Before John could reply with his own version of sarcasm, the world seemed to stretch and warp.

They had entered the wormhole.

Somewhere in Danville…

"Well, that action figure set with real fighting action didn't work out."

Ferb nodded at his stepbrother's statement.

Phineas Flynn continued, "Really? Why would they fight themselves to pieces?" He shrugged. "Oh well. I guess we should remember that."

Ferb gave him a thumb's up.

A sudden BOOM caused them both to look up. "What was that Ferb?"

Ferb noticed an object moving through the atmosphere. Something large, on fire, and heading for the harbor. "I'd say we should wait until it lands, then find out."

At Doofenshmirtz Evil, Inc…

"Perry the Platypus, don't you get it? I have no idea where those time-space holes go to or what might come out. For all we know, there might be aliens on the other side that aren't too friendly!"

Perry resisted the urge to slap Doofenshmirtz. While his nemesis was annoying, he did have a point, albeit a very unlikely one. After all, Perry actually knew some aliens who were friends with his owners, Phineas and Ferb. The likelihood of aliens coming in to invade, after all the boys had done, was slim to none.

The evil scientist suddenly gained a thoughtful look. "Do you think I could make myself their leader?"

This time, Perry did slap him.

Date: November 7, 2554

Location: UNSC Forward Unto Dawn

Personal: UNSC Artificial intelligence serial number CTN 0452-9 and Master Chief Spartan 117

Status: Crash-landing on unknown planet

"Chief, are you sure you won't strap in? It would just be embarrassing to die because you wouldn't put on a seatbelt."

"We'll be fine."

Staring out at the flaming trail the remains of the ship was leaving, he suddenly tensed.

"Chief?" Cortana asked. Then her processers caught up to the event. "Oh no. You're not really going to-"

He jumped.

It wasn't at all like the last time. This time, he was aiming for a body of water and was only about a kilometer in the air.

That didn't mean it was a soft landing.

The force of hitting the water knocked him about inside his armor. He shook it off and swam to the surface, but there were a few twinges of pain.

"Tell me when you decided freefalling was an acceptable landing procedure?"

The slight quiver in her voice was a bit of a shock. John didn't think it would rattle her so much.

"Back on Earth. It was the only way off Truth's ship."

He could almost see her exasperated expression. "So it wasn't enough that the Covenant were trying to kill you, and you had to try to do it yourself?"

"Thought I'd mix things up a little."

"Sure you did."

Danville Harbor…

The boys stared at the huge chunk of metal sticking out of the harbor. The fact that anything was visible proved the monstrous size of the object, seeing as that part of the harbor was a good 500 feet deep.

"So, what do we do? Go out there and check it out? Or let someone else beat us to it?"

Ferb rolled his eyes. When Phineas attempted sarcasm or rhetorical questions, it generally wasn't that good. "We check it out, of course."

Date: November 7, 2554

Location: UNSC Forward Unto Dawn

Personal: UNSC Artificial intelligence serial number CTN 0452-9 and Master Chief Spartan 117

Status: Crash-landed on unknown planet

John was just climbing back into the wreck when Cortana said, "Chief, I've got some good news and some bad news."

He waited for her to continue.

"I've accessed a wireless network on this planet. The good news is that we're on Earth."

John immediately straightened slightly.

"The bad news: we're apparently over 500 years too early."

Date: Fortieth Age of Conflict

Location: Supercruiser Blade of Judgment

The Prophet of Judgment looked out over this planet. It appeared to be the one called Earth, where Truth had vanished many cycles earlier. Yet, there were no defenses, no ships to combat, and no artifact revealed.

But there were plenty of humans.

Cleansing the planet would have to suffice.

There it is, the first chapter. What do you think? Good? Bad? Just plain ugly? Stupid beyond all reason?

I'll take anything in the reviews. Just, if you flame me, please tell me why.

Also, the date/location thing is first chapter only. It'll be gone next chapter.

So, review!