Now this here damn near broke my heart...

Just to note, if you watch the 2003 series there is actually a few episodes based on Donatello because he actually did get mutated twice and into a beast. He got infected on the episode "Outbreak" and actually mutated on the episode "Adventures In Turtle-Sitting". There is a few episodes between and it shows Donnie gradually getting sick. After "Adventures In Turrtle-Sitting" there are the episodes "Good Genes Part 1 & 2" which shows the family trying to cure Don. You should watch these episodes as they're really good

I think you did very well with this! That was quite a challenge to undertake, and I think you pulled it off. I kept imagining his double mutation from "Good Genes" in the 2k3 show :). I like that effect; the making it ambiguous.

"My child," he said softly, "My poor, bright boy."

Yes, poor Donnie! But we still love him without a doubt! I cannot understand why this wonderful and excellent piece of work has only one Review. It deserves far more! So here I am!
It is sad and wonderful at the same time. I love it how you described the reactions of all of them. How Donnie didn't want them to look at him if he were a monster and how Mikey reacted when Donnie asked him what kind of nickname he chose. The answer: Big brother is brilliant. I love that! I love this story! You did a brilliant job here and I wanna thank you!

That was great! Sad, but great!