The doorbell rang. "I'll get it!" called Phineas, and he and Ferb ran to the door. He loved getting the door. There were so many possibilities. The person at the door might be an amnesiac hobo who was secretly the heir to the throne of Druelsestein. Or a lost astronaut who thought he was on Mars! Or...

Phineas opened the door. Then he looked up and down the street. "Huh. That's weird." There was nobody there.

He turned to look at Ferb, who pointed down.

"Oh!"

A little platypus stared up at them from the doorstep.

"Hey, little guy," said Phineas. He picked him up. The platypus was a bit lighter than Perry, and his fur was a bit darker, too. "Where'd you come from?" He and Ferb looked out the door. Yes, the street was definitely empty. Phineas shrugged, and they carried him inside.

A note fell to the ground. Ferb picked it up, read it, and handed it to Phineas.

"Please look after this platypus," Phineas read. Well, it worked for Paddington Bear. "I bet Perry would love a playmate!" The platypus he was holding jerked a little, which he took as encouragement. "Come on, I'll introduce you!" He looked around. "Hey, where is Perry, anyway?"

Ferb pointed to the living room, where Perry was sleeping in his basket.

"Oh, there he is!" Funny, Phineas would have expected him to have wandered off by now.

.

Perry dozed. He was just awake enough to know that he wouldn't have to get up any time soon. Finally, a day off! Even with the best job in the world, it was nice to be allowed to relax sometimes. Today, he didn't even have to think about evil schemes, or implausible martial arts moves, or not blowing his cover. He'd even got Doofenshmirtz to sign a statement that he would not try to hurt anybody or do anything to the Tri-State Area until tomorrow, and that hadn't been easy.

He became aware that there was somebody nearby, and sleepily opened his eyes. Then he jumped. Phineas was putting another platypus down next to his basket.

"There you go, little fella!" said Phineas. To Perry he said "We found him on the doorstep! Cool, huh?"

Perry stood up on all fours and inspected the other platypus. He was a bit smaller and skinnier than Perry was, and he stood with his legs slightly bent. He also seemed to have some brown hairs scattered through his fur, which was weird. Perry had never heard of a platypus with brown fur. The stranger stared at Perry with a weirdly intense gaze for a platypus.

Perry made eye contact and growled. He had nothing against the stranger, but it was the proper protocol when another platypus entered his territory. Not growling would be like saying that it was okay to come into his place and take all his stuff.

Instead of acknowledging his dominance or even challenging him, the other platypus turned away and inspected Perry's food bowl. Perry stared. He felt like he'd just taken a breath underwater. That shouldn't have happened. The stranger was either very, very rude, or very, very naive.

Phineas followed his gaze. "I guess he's hungry," he said to Ferb. "Wait here!" he told the platypus.

Once Phineas and Ferb had run to the kitchen, the platypus turned back to Perry and said "So, how am I doing?"

That was Doofenshmirtz's voice.

Perry froze.

Oblivious to his reaction, the platypus - Doofenshmirtz - continued. "Hey, you know what? My nemesis is a platypus with exactly the same name as you! Exactly the same! I mean, what are the odds?"

Perry's rising panic subsided very slightly. Doofenshmirtz didn't recognise him. Not all was lost. But Doofenshmirtz was still in his house. Doofenshmirtz couldn't be in his house. What was he doing there? The boys were in danger.

"I suppose maybe you're wondering what's going on and how..." He hesitated. "Or maybe not? You know, I did some research before I came here, I watched a nature documentary... It turns out platypuses aren't very smart." There was a brief pause. "I know, I was surprised too."

Perry gave him his best vacant stare.

"Oh, what the heck," said Doofenshmirtz, with a very human flip of his hand. "So like I was saying, my nemesis, he's a platypus. Every day I come up with a new evil scheme, and every day he thwarts it without even breaking a sweat! Oooh, it makes me so angry sometimes! So I said to myself, Doof - that's my name. Sort of - Doof, your problem is that you don't know any of his secret weaknesses."

If Perry had any secret weaknesses, then they were so well-hidden that not even he knew what they were.

"I mean, he's got to have some, right?" continued Doofenshmirtz. "Everyone has... Anyway, like I said, I watched a nature documentary about platypuses, but they spent nearly the entire time just talking about how "weird" and "confused" they were, and it's like, they know what they are. You're the one who's confused. And then it came to me. Instead of watching some smarmy nature presenter make things up, I could just turn myself into a platypus, go to the house of someone who I know has a pet platypus and observe their behaviour. I... I think the rest is fairly self-explanatory. I left a note. You know, it worked for Paddington Bear..."

Perry turned and took a bite of his food, partly because he was hungry, but mostly so that Doofenshmirtz couldn't see his face. He wasn't sure how long he could keep up the vacant expression without his true feelings slipping out. How could he know that Phineas and Ferb had a pet platypus? Had the platypus themed restaurant they'd made had been a clue? The restaurant had been harmless, so Perry hoped that that was all it was.

He swallowed and took another bite. Technically, Doofenshmirtz had not violated their agreement. He wasn't trying to hurt anyone or take over anywhere. But he didn't like Doofenshmirtz being so close to Phineas and Ferb. It frightened him. And the longer he stayed here, the higher the chance that he might discover who Perry really was. And he didn't like Doofenshmirtz being so close to Phineas and Ferb.

Perry realised that his thoughts were going around in circles. He needed to clear his head. He needed orders.

Perry turned and walked towards the back door, willing himself to walk slowly. The side of the house entrance in the backyard should be safe at this time of day. He hoped.

"Oh, you're leaving?" said Doofenshmirtz, behind him. "Uh, okay! See you later!"

Perry made sure he'd left the house before he let any expression show on his face. Once outside, he relaxed into a worried frown. He really didn't want to leave Doofenshmirtz alone for too long, but he had to tell headquarters what was going on. Maybe they could send in someone else in to take care of the problem... No. That would look even more suspicious. But he had to tell headquarters.

.

Perry arrived at his base, put on his hat, and turned on the screen. A giant image of Major Monogram appeared. He looked a little confused. "Uh... Agent P. You are aware that it's your day off, right?"

Perry nodded impatiently, and quickly and concisely explained what had happened.

Major Monogram looked alarmed. "Great googly moogly! Do you think he suspects anything?"

Perry shook his head. Unless Doofenshmirtz was playing dumb, which was unlikely, he wasn't going to recognise Perry unless he was wearing his hat. It would have made Perry think that all platypuses looked the same to humans, but he knew that Phineas could recognise him in a group. So it was probably just another thing about Doofenshmirtz.

Major Monogram rubbed his chin. "Well, all the agents are out on missions, so... I'm afraid you're on your own for now, Agent P."

"I could help him, sir!" called Carl from offscreen.

Major Monogram looked to his right. "I already told you, Carl, no more missions until the Slushy Corporation accepts our apology!"

"Aoaw..."

That was probably for the best. Doofenshmirtz would have recognised Carl. Sometimes Perry wondered whether the annual good/evil mix and mingle events were such great ideas, but he was just a field agent. He couldn't hope to understand these things.

"I'll see what I can do," said Major Monogram, "but for now, your number one priority is to prevent Doofenshmirtz from discovering your identity."

Perry saluted. It wasn't much of a goal, but it was enough to make him feel a lot better.

.

When Perry returned to the living room, he found Doofenshmirtz sitting in front of the TV, watching car ads with a confused slant to his tail. Luckily, there was no sign of Phineas or Ferb. Perry watched him for a while, but he didn't seem to be doing anything evil. Finally, he chattered to get Doofenshmirtz's attention.

Doofenshmirtz looked over his shoulder. "Oh. It's... it's you. Hey, let me ask you something. What is ithis/i?" He stood up on two legs, with a much less pronounced slouch than usual, and waved his hands around.

Perry blinked once.

Doofenshmirtz tapped his bill. "I think it's coming from my beak. I've got a... I've got a funny feeling in there." He walked out of range of the TV's electrical field. "But now, it's gone!" He walked back to the edge of the field. "Here." He turned away from the TV. "Gone! Weird, huh?"

Perry stopped himself from rolling his eyes. It occurred to him that maybe humans couldn't actually sense electricity with their mouths. He'd never really thought about it before, but it made sense. It wasn't a very useful ability out of water. And it would explain why Doofenshmirtz called it a feeling when it wasn't. Human languages didn't include many words for concepts outside their experience.

Out of the corner of his right eye, Perry noticed Candace walk by. Doofenshmirtz had his back to her. Perfect. Not wanting to give away her presence, Perry very deliberately didn't look at her.

"I tell you, Perr-" began Doofenshmirtz.

Candace gasped. "Did you just talk?"

Doofenshmirtz jerked in surprise, and quickly turned around.

"Haven't I seen you somewhere be-" he said, then guiltily dropped to all fours and made a passable attempt at a chattering noise.

"Oh no," said Candace, glaring. "That's not gonna work on me. Just wait until mom finds out that Phineas and Ferb have genetically engineered a talking platypus!"

Seeing that Doofenshmirtz was about to run, Perry accidentally trod on his tail.

"Argh!"

Candace picked Doofenshmirtz up and carried him outside. Perry followed.

Phineas and Ferb were sitting under the tree, intent on some blueprints.

"You guys are so busted!"

The boys looked up in puzzlement. "But we haven't even started construction yet, Candace," said Phineas.

"Ugh, not... whatever that is!" Candace indicated the blueprints with her free hand. "This!" She thrust Doofenshmirtz forwards. "Genetically engineering a talking platypus? That's weird even for you guys."

"He talks?" said Phineas, and he and Ferb stared at Doofenshmirtz. "Awesome!"

"Yeah, and I'm gonna go straight to..." said Candace, then paused. "Wait. What do you mean 'He talks'? You're the ones who made him!"

"Listen, Candace," said Phineas. He walked over to her. "As much fun as that sounds, we didn't create him. We just found him on the doorstep this morning."

Candace stood there for a second, then slowly turned Doofenshmirtz to face her. "But if you found him, then why can he talk?" Her voice was full of suspicion.

Perry noted with satisfaction that Doofenshmirtz looked terrified.