After stating that he was going to join up with Wyrm, he attacked the turtles using Wyrm’s powers. Casey had gotten Donny alone in an alleyway.  He pushed him to the wall and pinned him there. 
“I know everything now Donnie,” he leaned in close to his face and whispered
“I know how you feel about me.” Their lips mere inches apart
“I feel the same way.” He pressed his lips into Donnie’s
Donnie closed his eyes and a shiver ran down his spine. He never imagined that kissing Casey would feel so good. Casey pulled away.
“When this is all over, I need you to try to hold on to one single thought.”
Casey pulled up to the side of Donny’s face by his ear slit.
“Tell him.” Casey pulled away from him. 
Don was a little more than shaken, first Casey betrays them for Wyrm then he makes out with Donny. So naturally, D was a little more than confused. But he didn’t have time to mull any of this over. Not now that Casey had returned to his crazy Wyrm induced attack.

When Casey had been granted one more wish and wished he never found Wyrm, Donnie finally understood.  If he ever wanted to have anything more with Casey then friendship, he was going to have to use all his mental power to remember Casey’s Words. Casey gave him one last look before the impacts on his wish took hold. Meeting Donnie’s eye he mouthed the words 
“Tell him.”

Working in his lab, in a rare moment of peace, Donatello couldn’t help but feel like something was amiss. As if some ghost from their space adventures had followed him home and was hunting his mind.
He sighed realized that nothing would be accomplished if he didn’t quiet his mind. He still sitting at his desk, he slowed his breathing and closed his eyes trying to meditate.
 In the quiet of his mind, he heard “Tell him,” echoing in Casey’s voice no less. A strange yet somehow familiar shudder ran down his spine.  He opened his eyes quickly, and panic set in.  
~Tell him?!~ He thought to himself ~is that a joke. ~
~Casey’s a guy…. A human guy and I’m….. ~
Donnie lowered his head as sad expression seeped into his face. He ran hands over his face as if trying to push the sadness away.
~Beside Casey likes April, End of story! ~
Donnie tried to focus on the projects before him. Anything to keep from thinking about his feelings for Casey.
“Tell him.” Casey’s voice ring in his mind again and Donnie jumped up from his desk chair
“No! I won’t! I can’t! I….”  he whispered out loud  
“TELL HIM.” Casey’s voice was loud enough to sound like he was shouting in his head
“STOP IT CASEY!!!” Donnie shouted hoping to silence the voice in his head
“Stop what Dude?!”
Donatello turned around
“I just got here,” Casey said as he stood in the doorway smirking. Donnie stared back at him in stunned silence for a moment. Casey stepped into the lab, his smug face turning to one of confusion.
“You ok D? you’re acting weird dude.”
Donnie shook his head and brought himself back to reality.
“Yeah, I’m fine…I just… I’m a little tired.” He said
“Not too tired to go to the junkyard though, right? We gotta get those parts, remember?” 
“Oh right…”
“You sure you’re up for this D?”
“Yes of course.”
Donnie pushed past Casey made his way to the doorway, he stood in the doorway.
“Let’s…”
“Tell him.” Donnie heard in the softest whisper
Donnie sighed and hung his head. He grabbed the door and held it in his hand, unsure if he was making the right decision but now fully aware that the voice would not let him be otherwise. 
“Casey…before we go…. I gotta tell you something.”
Donnie shut the door and made sure to close the door firmly.

They never made it to the junkyard.

Don wanted to make sure that Casey understood that he was under no obligation to act on Don’s feeling and in the middle of saying that Casey had cut him off by taking possession of his mouth.

Donnie was more than a little startled and would have taken a step back to question Casey about actions, but Casey’s hands were on the side of his face holding him in place.

Donnie then decided instead of questioning Casey’s Action, to just sit back and enjoy them

Casey’s lips were softer than Don had expected them to be. Casey flicked his tongue over Donnie’s mouth and Don’s lips open automatically in response.

“mmhmm” Donnie moaned tasting his tongue.
The kiss lasted a couple of minutes before Donnie suddenly fell back on the infirmary cot. Casey had pushed them there during the kiss. Bewilder Donnie looked up at Casey.

“How?... When?” He gasped as he realized Casey Jones was stripping 
“Wha-what are you doing Casey?”

Casey smiled wickedly at the turtle as he stood there shirtless with his pants around his ankles.

“I like you, you like me, So what’s the problem?”

He leaned forward towards a nervous Donnie who in returned scooted back on the cot.

His carapace hits the wall and it was then that Donnie realized that some way or another during the kiss Casey had stripped him of his weapons staff, his other weapons, his tech basically everything but his elbow pads, knee pads and mask.

Casey was more of a ninja then any of them had realized.
 He was a damn sex ninja.

Donnie put his right hand up to stop Casey’s advances and it landed on his chest.
“Casey… shouldn’t we talk about this first? Like about our relationship? about the future before we take this step?”

Casey took Donnie’s hand firmly in his own

“I ain’t much for talking,” Casey said he bit his lip as he said that staring hard into Donnie’s eye
“But actions…” he then moved Donnie’s hand towards his mouth and began to suck each of his fingers.

Donnie gasp and his heart raced

Casey kissed his way down Donnie’s arm to his elbow pads which then he removed while making hard contact with Donnie again. Once the arm pad was removed Casey, still gripping his arm, moved his face closer to Donnie’s slow at first and then quickly switched gears and bit down hard unto Don’s shoulder.

Don’s groan was loud, and he blushed and slapped his left hand over his mouth.

Casey grinning wildly let out a dark chuckle then proceeded to kiss his way down Don’s Plastron.

There was a bulge at the bottom of Don’s Plastron.

Casey stroked it lovingly.

“Ya gotta help me out here pal,” Casey said his voice deep and husky
“not sure how to get this little guy out his shell.”

Casey then started kissing and nipping Don’s thigh all the way down to his right knee pad which he then removed. When he moved to the other leg, he bit Donnie as hard as he could with his front teeth missing, after he removed the left kneepad and suck and kissed his way back up to Don’s thigh. When he reached the bulge of Don’s plastron, Casey looked up at Don expectantly.

Don was heavily panted when he looked down at Casey. Not wanting things to end there. Donnie reached down stroke the area in which his penis lay hidden.

Casey stared in amazement at seeing the plates open revealing a pouch and then Donnie’s manhood sprung into full view. And in a flash, Casey’s mouth was on him. Licking and sucking Donnie as if his cum held the secrets to life itself. Donnie’s slammed his head against the wall with his eyes shut tight Casey’s tongue flicked over the tip of Don’s dick and his toes curled.

Donnie opened his eye to see that Casey still had his underwear on. Don took off his other elbow pad his last stitch of clothing remaining. Using both hands to gently yet firmly push Casey off him and down on to his back. Casey stated back at him, eyes wide. Donnie then turned his body, carefully so not to hurt Casey, to where his erection now hung down in front of Casey’s face. Donnie slides off Casey underwear with ease and tossed them to the floor. Casey then smirks realizing the position Donnie was putting him in. Donnie’s mouth was only inches away from Casey’s member when without warning, Casey flipped Donnie on to his carapace and then straddles him. 
“I’m on top!”  Casey licked his lips looking at Donnie’s thick dick
“Really Casey? Does it honestly even matter? I mean if you look at the position, we’re in it really doesn’t matter who’s-”
Don was abruptly shut up by Casey shoving his dick in Don’s mouth.
“Doesn’t that mouth have better things to do right now D?!”
Donnie had some Choice words for Casey, but they all died when Donnie realized that his tongue was tasting Casey’s flesh.  Casey sucked Don hard while Don earnestly sucked him back. Both driving each other closer and closer to the edge until they finally climaxed into each other’s mouths. Casey bucked hard into Don’s mouth as he came, and Don opened his throat wide swallowing all of Casey’s semen.

Casey for his part gobble Don’s cock down clamping his lips at the base not wanting to lose a drop. When both were sure that the other was finished, they release each other panting heavily.

When Don thought he had his breath under control again he turned to Casey. 
“so…we going to talk about us now or what?”
Casey grabbed the covers at the bottom of the cot and pulled them up with him to meet Don’s face.
“Later dude.” Casey wraps his arms around Donnie’s neck, kiss his cheek and falls asleep with a big dumb grin on his face. Don sighs and looks up at the ceiling.
“This idiot’s lucky I think he’s cute.”