The average platypus can live a maximum of about fifteen years in ideal conditions. And Phineas and Ferb had done extensive research into the way to raise a platypus, so they expected him to live for about that long.
Of course, that estimate doesn't exactly take into account the stress of epic battles of good versus evil. Having one of those every day tends to cut the time down. For a platypus with such responsibility, ten years is a relatively long time to live.
The first one to notice was Major Monogram, but he didn't do anything about it. Regrettable as it was, it had always been expected, so he simply let it be. Fortunately, Perry didn't seem to notice, so that made things a lot easier.
Linda Flynn was the second person to notice, and her reaction was a talk with her kids. They had grown and were mature enough to understand, but that didn't do anything to quell their sadness. They began to sit with him on a daily basis and cuddle. Of course, since they didn't know that he'd be able to understand them, they didn't bother to hide the fact that it was happening.
And suddenly everything that had happened recently made much more sense.
The way that he had become unusually slow to catch his breath. The uncomfortable awkwardness of the agents around him. And now that he knew what he was looking for, it seemed obvious. Hadn't his fur had become a duller shade of teal recently, wasn't his memory playing tricks on him, hadn't he caught himself absentmindedly looking for peaceful places as a part of some animal instinct occasionally? He could barely believe that he hadn't noticed it before.
After all, ten years was a relatively long time for a platypus leading his kind of life to live.
He retired in hopes of stretching out his last days into a few weeks and let himself be pampered by his concerned owners. Monogram instantly consented and offered him a bonus. Perry instructed him to give that bonus to the Flynn-Fletchers over time in ways that they wouldn't notice anything out of the ordinary. He seemed a little put off by the difficulty of the request, but Perry knew Monogram couldn't refuse his best agent.
He did end up successfully living for another ten days, which was less than he'd hoped for but more than he'd expected.
And in the end, it was time well spent, cuddling up with his owners and, for the first time in almost his entire life, just spending time with them. He even got to witness them creating a few of their ever-famous inventions, including a machine with an expert ability at rock-paper-scissors, a system for remembering everything, and (at the request of the Fireside Girl, Isabella) an obstacle course with first-aid activities as the obstacles. Candace, who was back from college for summer, had given busting up for the time-being and resigned to simply letting herself join their fun, and it was nice to see her getting along with her siblings.
Of course, all good things come to an end.
When he woke up in the middle of the day, he knew he didn't have much time left, so he set to do two more things, two extremely important things. For the first he traveled down to his lair and the room with various reconstructed inventions. He looked at the boy's old things fondly before turning to Doofenshmirtz's –inators, looking in particular for one.
There it was, in the corner of the room. The amnesia-inator. He flipped the reverse switch, wondering at Doofenshmirtz's decision to include such a switch. It seemed like common sense that it would detract from his plan. Then again, Doofenshmirtz tended to occasionally lack any sort of common sense when it came to his –inators.
In any case, he adjusted the settings for the correct range and activated the –inator.
Elsewhere, a Phineas sitting on the couch who had just begun to attempt a smile suddenly burst into tears. He clutched his brother's shoulder and his body began to wrack with heavy sobs, as Ferb wrapped his arms around him even as tears began to fall from his eyes. Candace watched from a distance, not needing to ask what was wrong and not knowing how to help, how to even try to comfort them other than slowly approaching her and placing a sympathetic hand on Ferb's shoulder. Both boys responded by half-tackling her in a tight hug as they just sat on the couch and remembered.
With that done, there was only one thing left. It was almost funny that even with his mental capacities slowing toward the end, he knew the last place he wanted to be in an instant. It was hard to believe that such a battleground, such a place of traps and threats and fights and explosions and curses was his chosen peaceful-place, his chosen final moment. But then again, not really.
After futzing with the controls for a minute or so, he gave up on his jetpack and walked to the building.
The building that had slowly become his second home.
The building.
He tried for a few minutes to at least get his hookshot working before approaching the door and pressing the button to call to Heinz's room.
"Who iiis it?" a familiarly singsong voice said cheerfully. Perry chirred into the intercom and waited for a response.
"A platypus call?" the voice asked, and for a second Perry stared in confusion before realizing his mistake. Rolling his eyes, but nonetheless amused, he retrieved his hat and placed it on his head before trying again.
"A Perry the Platypus call! Just a sec, Perry, I'll buzz you in." Not letting himself think too hard about how his hat mattered over an intercom, he waited until the door was unlocked and walked in, taking the elevator up to the top floor.
As soon as the bell dinged and the elevator doors opened, Perry found himself face-to-face with the man and caught himself smiling slightly.
"Alright, Perry the Dense-ypus, you've got some explaining to do!" the man said in that familiar and nostalgic voice, "Where have you been and why didn't you call once?"
When Perry didn't respond other than looking down, the man's irritation quickly grew. "I mean it! I was worried sick, you know? You're always assuming you can just blow me off to deal with some other risk, well-well you know what? I'm a…a…uh…"
The man trailed off once he looked directly at Perry. "Say, Perry the Platypus, are you feeling okay? What, are you sick or something? You look green…er."
Technically, he was right, so Perry slowly nodded, and, though Heinz's anger did not seem to fade, he at least redirected its target from Perry. "The agency is making you come out here sick? And they call me evil, sheesh."
"Well, with your time off, I haven't really bothered making a scheme recently, so you don't need to thwart me. But-but don't feel bad or anything. Even someone as 'I'm Mr. Emotionless' as you can't help getting sick sometimes." His lack of serious concern made Perry cringe as he realized that nodding about sickness was probably a bad idea.
"Hey, c'mon, I'll make you some of my famous chicken soup. It's a family recipe!"
As little respect as he had for Heinz's family, Perry had to admit that they were a group of culinary prodigies. And he couldn't turn down an offer like that, even if chicken soup wasn't going to exactly cure him. He followed him into the kitchen and sat at the table as Heinz put some ingredients he didn't bother to watch into a pot on the stove. The man sat down, grinning casually with just a hint of spite.
"There's actually quite a story behind this soup. It's from my uncle's third cousin's grandfather. I only managed to inherit it through a few contrived coincidences and some plucky wit on my part. It all started…"
As Heinz continued to ramble, Perry tuned him out and wondered how he was going to breach the subject. A language barrier that could not be passed separated him from the man, after all, so he had little way to tell him anything, let alone something this delicate. Typically, he might just mime it and let comedic misreading take its course, but he couldn't afford to leave this misunderstood for even a moment more.
"Hey! Are you even listening to me? You're so rude, you know that, Perry the Platypus? All my life, I've faced people belittling me. And even you do it too! What a world."
Maybe there was some ringing truth behind Heinz's typical and hypocritical complaint. Maybe it was just the platypus's emotionally compromised state. But, either way, some nerve was struck and he twitched and suddenly tears were rolling down his cheeks and any attempts to maintain stoicism suddenly became ridiculously impossible.
Heinz's face quickly softened and filled with concern. "Whoa, whoa, whoa, what?! P-Perry the-Perry the Platypus, really, what is going on?" The man began to panic, suddenly looking guilty. Perry hopelessly rubbed at his eyes, but the damage was done and even if the tears would stop (they wouldn't, of course) there was no way to undo them.
"Look, you have to-" The stove timer beeped loudly, interrupting Heinz and uncomfortably making a sheepish attempt to dispel the tension. Heinz continued to look at Perry for a few moments before finally getting up to turn off the timer and the stove, directing constant concerned glances toward Perry. Glad to have been given a few moments, Perry used this as a chance to collect himself and stop the tears.
Approaching the table, Doofenshmirtz put a bowl of the soup in front of Perry, calmly saying, "Perry the Platypus, please tell me what-"
Quickly terrified by the idea of confrontation on this topic, Perry began quickly and frantically slurping the soup loudly. After recently crying, he could barely taste it, but he was willing to sacrifice that so that no conversation would commence.
Even if he had come to the building ready and unafraid, he hadn't anticipated how hard it would be to actually say goodbye.
"Perry the-!" A loud slurp. "Look, j-just-!" Another one.
Finally tired of it, Doofenshmirtz yelled, "Perry!" The use of his title-less name caused Perry to freeze.
"L-look, I don't know what's going on or why you're acting so weird b-but I, I want to, so tell me."
Perry looked away, at which point, Heinz reached across the table and put his hand on the platypus's, smiling so gently, so genuinely.
"Who knows how often you've helped me, Perry the Platypus? Let me help you for once."
Perry cursed the fact that Heinz had chosen this as the day that he would become assertive. He breathed a sigh before, trying to smile but failing, he nodded.
Still uncharacteristically calm, Heinz walked into the other room for a few moments and returned with a pen and a blank white sheet of paper. "J-just try to write down what's going on for me. Take as long as you need."
He took another deep breath and glanced at Heinz in some sort of begging for reprieve before clicking the pen and beginning to write.
There was a lot of crossing out and using carats to add a detail and crossing those out, but in the end, it came out to about one short sentence, which he looked at for a minute without editing before finally handing it to Heinz and feeling the permanency set in as Heinz read it.
As he'd expected, Heinz's face turned white and he looked at Perry as if hoping he'd misinterpreted it. When Perry just looked away, he reread it again before sinking into his chair. "O-oh. Oh."
They were silent for a long while, with Perry looking away uncomfortably and Heinz trying desperately to say something but being unable to think of anything worth saying that would not serve to ruin the artificial calm that their mutual awkwardness had somehow created.
The platypus sniffed almost inaudibly, at which point Heinz just gave up on any semblance of composure and pulled him into his arms across the table and hugged him tightly, too tightly as if he was never going to let go and never wanted to spend any more of his moments separate from him. It would seem like the typically overbearing moment between them unless someone looked closely and saw that Perry was holding on just as tightly.
They used no more words, written or otherwise, and simply held each other in that way, weeping softly and brushing hands against each other to keep their breathing steady until an emotionally exhausted Heinz found himself asleep and Perry, unwilling to risk waking him up, stayed as still as he could. He smiled gently when he saw Heinz wasn't twitching or groaning in a nightmare about his parents or childhood like he had seen when he had caught the man sleeping before.
Even if they still relied on a good-versus-evil fight as an excuse to interact, their relationship had grown so far beyond that. In particular, Heinz had grown so much. Maybe that's why a Perry with tired eyelids who felt weaker than ever was able to let himself fade away and fall asleep, feeling that in his short life he had done enough.
And two heartbeats synchronized before one of them faded and left the other solitary but not alone.