Donatello is sick so he is staying with Casey and April. However, he starts turning into one of the mutants that the Turtles will have to fight against.
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!!This episode provides examples of

* AccidentalPun: April notes that Don is looking green before realizing that this statement needs some more clarification.
-->'''April:''' Whoa. Don, you look greener than usual. A ''lot'' greener than usual.
* ActuallyIAmHim: Leo, Raph and Mike attack Monster Don at April's basement then wonder what happened to Don.
-->'''Leo:''' What was that thing?\\
'''Raph:''' And where's Don?\\
'''Mike:''' Did that monster get Don?\\
'''April:''' Guys, that monster ''is'' Don!\\
'''Mike:''' Say what?\\
'''April:''' That thing you just fought is Don.
* [[ArtEvolution Art]] and VocalEvolution: Leatherhead not only has lighter skin than before, but he now has a more pronounced accent.
* AsYouKnow: Leatherhead uses the exact words when explaining how he and Don made the containment unit. Since this is Leatherhead's first appearance this season, it's meant to be exposition for the audience.
* BackFromTheDead: Baxter Stockman, thanks to Bishop saving his brain and attaching it to a new cyborg body.
* TheBait: Mikey gets volunteered by the others to lure mutated Donnie towards the containment cell where he can be held until they figure out how to help him. Mikey is [[OhCrap not excited]] about this.
* BigBrotherInstinct: Both Casey and April are a bit annoyed, Casey even more so, when the Turtles ask them to take care for a sick Don, as this means their plans for a romantic evening is ruined, but they both decide to look after him anyway. Casey is the one who tucks Don in and covers him with a blanket.
-->'''Casey:''' Come on, sickly mutant ninja turtle, you better lie down before you fall down. ''(Helps Donatello lie down on the couch and then covers him with a blanket)'' There. Comfy?
* BigWhat: Mikey does one of these ''and'' a BigNo when Leatherhead says he can't cure Donnie.
* BigWhy: Stockman's reaction to being revived yet again.
* BrainwashedAndCrazy: While Donny's not quite brainwashed, he ''is'' mutated and certainly crazy enough to live up to the spirit of the trope — unwitting FaceHeelTurn included.
* TheBusCameBack: Leatherhead makes his first appearance since "Exodus".
* CallBack:
** All the way back to "Outbreak", which is where Don picked up the mutagen in the first place.
** Bishop bringing Stockman back in a new cyborg body is also a reference to "Insane in the Membrane".
* CantKillYouStillNeedYou: Or rather, Can't Let You Die, Still Need You. Even besides Stockman's general usefulness, Bishop ''really'' needs his help working on the outbreak virus.
* ChekhovsGun: The tunnel to April's apartment: introduced in "The Trouble with Augie".
* ContinuityNod:
** Stockman has to be resurrected from the dead after his failing clone body fell into the river and drowned in "Insane in the Membrane". Since that episode never aired during the series' original run, viewers were left confused as to ''why'' he had to be resurrected until it released on the "Season 4" DVD.
** This episode features April's basement apartment, which hadn't been seen or referenced since "The King", three seasons earlier. It even had the appropriate pieces of furniture in it, like Kirby's drawing desk.
** In the beginning of the episode, Donatello is sneezing like in "Return of Savanti". This is revealed to be one of the symptoms of his secondary mutation.
* CurseCutShort:
--> '''Raph''': I'm gonna take Bishop and shove my foot ''so'' far-
--> '''Leo''': Easy, Raph!
* DeathSeeker: Downplayed. While he's not shown actively trying to kill himself in this episode, Stockman is ''deeply'' unhappy to learn he had finally died but is now brought back indefinitely to continue serving Bishop.
* DownerEnding: Donatello is left a mutated, mindless monster that must be kept in a cell. Baxter Stockman is revived. He will spend the rest of the series tortured at being ripped away from the peace death finally granted him.
* HeCleansUpNicely: Casey in the suit and with his hair done.
* HulkingOut: Don mutates into a Hulking beast, even bigger than the already huge Leatherhead.
-->'''Raph:''' [[OhCrap Whoa, mama!]] [[LampshadeHanging Don's been taking his vitamins!]]
* {{Metamorphosis}}: Don into a monster.
* MoodWhiplash: The episode starts out like a rather normal mutated monster catching episode with Donnie seemingly just being merely [[SickEpisode sick]] at first... then Donnie starts ''mutating into a monster''.
* MyFriendsAndZoidberg: Mikey says they need to save Don and April. A very annoyed Leo then reminds him that Casey also needs help.
* MythologyGag:
** The idea of a Turtle undergoing a secondary mutation into a monstrous form has appeared in other parts of the franchise, including [[WesternAnimation/TeenageMutantNinjaTurtles1987 the Fred Wolf series]] and [[ComicBook/TeenageMutantNinjaTurtlesMirage Mirage's fourth volume]]. In fact, Don's monstrous mutant form strongly resmbles "Gameraph", Raph's secondary mutation from the Mirage comics.
** The 1987 series also had an episode of the same name.
* OpenMouthInsertFoot: While facing some mutants at a power plant, Leo briefly forgets that Don is not there with them.
-->'''Leo:''' Okay, Mikey, you take the left. Don, you cover me on—\\
'''Mike:''' Uh, Don's home sick from school today, Leo.
* PoorCommunicationKills: Downplayed in regards to April and Casey going on a date. April dresses up as a BadassBiker because she thought they were going on a romantic motorcycle ride, while Casey arrives at her apartment as a SharpDressedMan under the assumption they were going to dinner together, causing a lot of awkwardness between the two. But they realize the good intentions the other had and have a good laugh.
* ResistTheBeast: Donny's transformation into mindless beast is obviously painful, yet he's able to resist it for long enough to beg April and Casey to 'stay away from' him as he runs from the room, only to turn into a monster and start attacking them.
* TheReveal: Donatello had been infected by the mutant outbreak and is now a monster.
* ShapeshiftingExcludesClothing: Donnie's elbow pads, knee pads, and belt visibly tear off his body as he mutates.
* ShoutOut:
** Mikey saying, "[[WesternAnimation/ScoobyDoo Like zoinks!]]".
** The title is also a reference to ''Film/AdventuresInBabysitting''.
* SickEpisode: For Donatello... until it's revealed to be something ''much'' worse.
* TheSmartGuy: Leatherhead takes up Donnie's role as this in the team.
* UnwantedRevival: Bishop retrieves Stockman's brain and attaches it to a new cyborg body, needing him to work on a cure for the outbreak. Stockman is absolutely mortified that he wasn't left to rest in peace.
* VomitDiscretionShot: Discussed:
--> '''Donatello''': I just can't handle mutants right now.
--> '''Michelangelo''': Did you figure that out before or after you barfed in the Battleshell?
%%* WeNeedADistraction
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