A Non-Descript Cardboard Box
Perry slid gracefully into his seat in front of the big screen in his lair. Major Monogram was waiting for him, coffee in hand. "Sorry about the coffee, Agent P, I got in real late last night."
"Don't you mean 'early this morning,' sir?" Carl asked rather nasally from off camera.
"No, Carl, I was here at ten last night, and I haven't left since because of your camera incident!" He complained, "Anyway, Agent P, Dr. Doofenshmirtz was seen carrying a cardboard box into Doofenshmirtz Evil Incorporated yesterday. Unfortunately, someone couldn't get an angle suited to read the label—"
"—There were bats in that air duct, sir!" The title-only paid intern protested.
"That's no excuse, Carl! This is why I don't let you do field missions!" He turned to Perry, "You know the drill. Find out what he's up to and put a stop to it. You're dismissed, Agent P."
Perry saluted and slid on his hang glider pack, flying through the ceiling light on his way to his nemesis' lair.
Not five minutes after the teal-furred monotreme left, he was captured by one of Doof's various traps, and sitting on the relatively-clean floor of the supposedly-evil-scientist's lair, not really listening to the self-certified doctor babbling and rambling on his 'evil scheme monologue' Bored out of his mind, the teal mammal took out his deck of cards to play a game of solitaire, so it came as quite a surprise when the inator the diabolical doctor had built exploded seemingly on its own, along with his own trap.
"P-perry the Platypus! I didn't know you were a magician! Even so, you know it's not polite to blow up my inators before I'm finished with my backstory!"
The teal-furred platypus agent ignored the crazed, self-declared doctor when his gaze found the rather non-descript cardboard box Major Monogram had mentioned earlier.
Silently, he pointed swiftly at the box in the corner as he padded over to it, and Doofenshmirtz stopped his ranting to look over at where he'd been pointing. "Oh, this?" He pulled the box out so Perry could peer in. "These platypups were left on the doorstep. I brought them in out of the cold last night. I don't know much about platypuses…platypi…platypeople…oh, you know what I mean, so could you figure out what to do with them?"
Chattering wearily, he nodded and proceeded to take the box back to the Flynn-Fletcher backyard.
It was, of course, Ferb who spotted the box filled with platypups first, and tapped his step-brother on the arm.
Phineas soon became enthralled with their presence, and excitedly yet carefully raced to show their mother. Ferb followed behind him, and they both pleaded—Phineas with words, Ferb purely through his eyes—to let them stay.
"Alright, alright." Linda amended with a sigh, "It couldn't hurt to let them stay until we can find a refuge center for them."
Phineas' face lit up in a brilliant smile and cradled one of the platypups in his arms. "They're adorable, aren't they, Ferb?" The redhead asked as their mother went to find some of Perry's old toys in the attic. The green-haired young man nodded and gently petted one of the other platypups still in the box.
Perry chattered happily in agreement as well as he entered the room; even he had to admit that they were so darn cute, despite being such a handful.
…
Although all the platypups liked Perry, one of them in particular grew very attached to the elder monotreme.
The youngest, and by far the runt of the clutch, sometimes slipped past him into Doof's own lair without him knowing if he wasn't careful.
Once, he went on a wild search for his hat, and found him wearing it, the little platypup dwarfed in comparison.
Perry gave the platypus-equivalent of a laugh as he listened to the nutty doctor speak to the young platypus as if it were him, "Well, Perry the Platypus, I don't quite know how your hat got so big…my inator had nothing to do with size today…It must be your hat that grew…after all, it couldn't be you who got smaller, that's too complex." He turned around to rummage through his inator parts, "I'll see what I can do, though."
The elder of the monotremes padded over to chide the younger, then took his hat back and placed it on his head. When the self-proclaimed 'evil' scientist turned back around, he exclaimed, "Oh! Your hat's back to normal! Why is there a platypup in my lair?"
Perry, of course, tried to explain, but Doof didn't find it easy to understand, and almost all intellectual thought was lost in translation.
Eventually, he gave up with a sigh, and took the platypup by the paw and escorted them both back home.
…
The littlest monotreme wasn't the only one to be chided after that event. Major Monogram called it a 'case of inappropriate conduct' to have misplaced his hat, despite the irrefutable humor the young platypus had caused. The major ended up saying this through poorly-stifled laughs.
It wasn't long before a wildlife rescue service had volunteered to take the platypups in.
Phineas sighed, "It'll be sad to see them go, huh, Ferb?"
Ferb nodded sadly, and hugged his brother.
"I'm gonna miss them." The redhead declared.
"But I'm sure the place they're going to take good care of them. Platypus are endangered, after all." Ferb replied softly. "Or perhaps we'll see them again."
"Thanks, Ferb." Phineas said sadly, and his green-haired step-brother patted him comfortingly on the shoulder.
They walked back inside, and then it was Perry's turn.
Putting on his hat, he padded over the box and gave them a proud salute.
They grew restless now, anxious about where they would be moving to, but they settled down once he'd promised that he and the boys would come to visit sometimes.
So when the truck came to pick them up, they only chattered happily, and when the truck drove away, everyone—including Perry, who stood out of sight of the family, his beloved hat atop his head—waved goodbye as they disappeared off into the distance.