The how and why of Knuckles' being here, ruthlessly pounded by Tails, was completely unimportant. All that mattered was the mutual, ineffable sensation of joy with which the pair enthusiastically provided each other.
Where that sensation came from, however, managed to nag at Knuckles' mind as he panted and moaned on all fours in response not to stimulus, but an idea. He could neither hear nor feel Tails' grunting and breathing against his neck, their hips clapping together, the arms around his chest suppressing any struggle; but he could see them—he knew they were there and it turned him on.
He could squint as hard as he could and still see everything perfectly: his own helpless drooling figure bumping forward and back in perfect harmony with his friend's powerful thrusts as he towered over him.
What he could certainly feel was his throbbing prostate shocking his member: the all-encompassing tingling of the inevitable release welling up inside of him. Tails picked up his pace as if somehow in tune with Knuckles' approaching orgasm, speeding up relentlessly with every thrust until finally…
Everything faded out of existence, replaced by the tangible. Knuckles' senses and motor functions returned once more. He heard the ocean beneath Angel Island, whiffed the evening breeze, felt the semen on-
“Not again,” he groaned.
He felt his anus clench uncontrollably at regular intervals that influenced his still fully erect penis to twitch in harmony, swinging straight up and splashing back down on his belly in a small puddle: the most substantial of the individual collections of jizz that had jettisoned across his body.
He watched the glistening strands of tenacious cum bridge the gap between his belly and the head of his penis as it rose, and more fluid weakly ooze out of his dick onto the expanding puddle as it fell back into place.
The amusement died down with the stiffness of his dick. The twitching and the pleasure had ceased, and all Knuckles was left with was shame and vexation. He picked himself up off the grass at the base of the Master Emerald's shrine, and marched ungracefully to the nearest stream to rinse off.
This was the eighth time in three weeks, and the dream was always the same. Knuckles decided to break routine and hike further to the waterfall; a cold shower might do him some good.
As he followed the stream toward the waterfall, the white noise and the breeze became more distinct. It helped distinguish by feeling alone where Knuckles had stained himself, and for the first time, he felt it on his face. It was getting worse.
He finally reached the deafening waterfall and stepped right under it without ever breaking pace, gliding effortlessly through the torrential shower. It was cold and loud: just what he needed to keep his thoughts from stagnating.
Even that didn't work, though. He kept coming back to Tails, realizing he had never considered where the attraction had come from in the first place.
He thought back to when Tails had first insisted on moving to the island just a couple years ago. Both had been stubborn and immovable on their stances:
“You need someone to keep you company. Every time I see you, you're perturbed about something.”
“I don't need anything from anyone, least of all someone to bother me in my own home.”
and Tails was right in a sense, as much as Knuckles regretted to admit it. The only time anyone saw him, he had a reason to be agitated that was either forcing him out of solitude or invading his privacy.
Maybe Tails was right about the rest, too. Maybe isolation wasn't the way to live now that he had friends who cared about him as much as he did for them.
Knuckles sighed to himself and stepped out from his cold shower, wringing his dreads dry as he walked downstream toward Tails' workshop. He had to tell him.
One of Tails' additions caught his attention partway there: a water wheel in perpetual motion. He smiled to himself, recalling the sleepless nights in the first weeks of Tails' move, convinced that the kid was threatening his territory with machinery.
“You might be moving in,” Knuckles had cautioned him. “but you’re still a guest in my home. Disturb my fixtures,” he gestured towards the jungle before clapping a fist into his palm with startlingly audible force, “and you pack your bags. I'll handle the demolition personally.”
He had assumed that would be a total deterrent—that Tails would call it quits and just go home—but he had evidently spent too much time around Sonic to give up so easily. 
“Preserve the ecosystem,” Tails summarized in response. He brushed his thumb across his nose with a smirk, “No sweat.” Those adopted mannerisms on a smart mouth were almost more than Knuckles could handle.
Above and beyond his word, Tails had installed all manner of clean power sources for his workshop. Water wheels upstream, solar panels on the roof, wind turbines outlining the runway—the dirt runway—leveled with scrutiny and cordoned off with sandbags. His plane no doubt needed special tires to land on that terrain, too.
All that effort for Knuckles’ benefit, and not once did either give the other a reason to complain. In fact, they kept to themselves most of the time. That was why he liked him. Someone like Sonic would have driven him up the wall: moving all over, wanton disturbances and trouble always close behind; and that was admittedly what he had come to expect of everyone.
Tails’ airplane hangar crept into view, plain yet curious all the same. It was where he lived, worked, and parked his pride and joy. Knuckles let himself in through the door in the front wall facing the cliffside runway.
His clanging footsteps echoed inside, as did his frustrated sigh when he saw that the plane was gone. He would have to wait, and he had already spent most of his patience anxiously walking here.
Knuckles trudged through the hangar, weighing his options. He had to get this off his chest now lest he wash more off his chest later. The sun had fully set, and the Master Emerald… was on a viewscreen.
He had completely forgotten about Tails’ security network of hidden cameras, but there it was: an entire desk stacked with columns of monitors spying on the island. By far the largest of them sat in the center of the mass, displaying the Master Emerald which shone so brilliantly that even the high-tech camera’s night vision toiled to maintain a consistent picture. It was hardly ideal, but he could watch the Emerald from here while he waited.
Knuckles’ relief was short-lived, however, turning right back to anxiety with the realization that he himself had been under watch all this time. He wouldn’t have normally cared, but how much had Tails seen? Or heard? Did he talk in his sleep? Did these things even have sound?
Whatever. He had come to talk about it, and that was what he was going to do. Eventually.
Hours.
Minutes.
More minutes.
Ticking seconds.
The hangar rang with Knuckles’ arhythmic footfalls, plodding around aimlessly.
Tails still isn’t back. Probably won’t be until morning. At least. How’s the Master Emerald doing? Why tonight? What to do.
He paced all about the enclosure on autopilot, thoughts racing in uncontrollable zigzags until he had tired himself out and was splayed face up on the ground catching his breath.
The metallic floor cooled his body from its own built up heat, normalizing his breathing and heart rate. He made himself comfortable there, lifting his dreads to better chill the back of his head, and took the moment to appreciate his fresh perspective.
This time would be better spent deciding how best to approach the conversation. All the same, Tails’ reaction was ultimately of little import. Knuckles turned and lifted his head toward the monitors. If he didn’t know already, he would find out on his own soon enough.
He let out a sigh, turned his head to one side to cool his face, and took note of the scattered mess of objects littering the floor. Wherever Tails had gone, it must have been urgent; he always put his tools back where they belong.
Knuckles turned to the other side, and noticed a new point of interest: a large doll under the bed. His eyes narrowed, and he crawled over to yank it out into the light.
His mouth fell open. It was him. Kind of. It had his size, his color, a crescent on its chest, even matching dreadlocks that were probably fashioned out of a mop head. What was oddly out of place, though, were its clothes. Ignoring the pair of mismatched oven mittens on its hands, it sported a transparent crop top and a pair of short shorts.
Knuckles should have been creeped, and he might have been if the crude facsimile had any more of a face beyond two violet dots for eyes; yet after the initial shock, he only felt relief once more, and proceeded to undress his shoddy clone.
“You should really stick to machines.”