The Turtle team hunts for their Sensei, but soon realizes they are being hunted as well.
- Agony of the Feet: Splinter's injured foot leaves him struggling. The Rat King also simulates what the Super Shredder did to him last time.
- Attack Its Weak Point: Karai tricks Super Shredder into getting in close by appealing to his twisted love for her, then punches his exposed heart.
- Bad Boss: Shredder is mentally deteriorating so much that he's even manhandling Tiger Claw.
- Badass Boast: The Rat King while attacking Splinter in his One-Winged Angel form.Rat King: You cannot rid yourself of me. I will be with you always in the darkest parts of your mind. I am your teacher! I am your MASTER!
- Be Careful What You Wish For: Bored while waiting for Donnie and Mikey to find Splinter, Casey takes practice shots and remarks he'd like for something crazy to happen. He ends up hitting Bebop, exposing his and Rocksteady's presence and triggering a fight.
- Big Bad Ensemble: This is the only episode where both of the most personal threats to Splinter (Shredder and the Rat King) antagonize him. Super Shredder/Oroku Saki and the Rat King/Dr. Victor Falco never meet, however, as the Rat King is a hallucination/ghost at the bottom of an abyss and his decayed body appears down there, while Shredder remains on the surface to confront Leo, Karai and April.
- Big "NEVER!": Splinter's reaction to the Rat King saying he belongs to him.
- Big "YES!": The Rat King when Splinter's feral side starts emerging.
- Body Horror: The mutation takes a toll on Super Shredder, as his arm starts withering.
- Body of Bodies: The Rat King surrounds itself in rats to create a giant rat monster, although it later turns out to be just a hallucination.
- Blood from the Mouth: Or rather, mutagen from the mouth; After Karai punches his exposed heart, Super Shredder coughs up a decent amount of mutagen.
- The Bus Came Back: The Rat King makes his first appearance in over a season... but only as a hallucination and a decayed corpse.
- Cerebus Syndrome: Easily one of the darkest episodes, being one of the only ones that actually shows death and blood.
- Dead All Along: After being tormented by the Rat King throughout the episode, Splinter comes across his corpse, confirming that Victor Falco did indeed die. Splinter comes to the conclusion that he was suffering from a fever dream, though it remains unclear if that is indeed the case or Splinter actually saw a ghost.
- Even Evil Has Loved Ones: Downplayed. The Super Shredder still wants Karai to call him father, but he still nearly crushes her with a truck during the street fight.
- Evolving Credits: The opening titles are altered slightly, replacing Shredder with his mutated form.
- Family-Unfriendly Death: While not seen directly, the condition of the Rat King's corpse leaves little to the imagination: his rib cage shattered and visibly tore out of his chest through his clothing from the impact and it appears the bones of his left arm tore through his skin and cloak.
- Faux Affably Evil: The Rat King offers to help his "old friend" survive in the pit.
- Flashback: To Mutation Day.
- Fire-Forged Friends: Starting to be this way with Karai and April.
- Flashback with the Other Darrin: Leo's line in the above flashback is done by Seth Green instead of Jason Biggs.
- Friend to All Living Things: Rat King advises Splinter to chow down on a rat to sustain himself, but Splinter can't bring himself to do it. Rat King derides this bit of nobility and says it will change after enough time in the pit.
- I Can't Sense Their Presence: April mentions she can't sense Splinter, though she states this could simply be because he's unconscious.
- Irony: The scene where Leo, Karai and April fight Tiger Claw, Karai saves April from a blow by Tiger Claw, considering not that many seasons ago Karai was the one trying to kill April and the two were enemies at the time.
- Little "No": Splinter when faced with the giant rat monster.
- Killed Off for Real: This episode confirmed Dr. Victor Falco/The Rat King died when he suffered his Disney Villain Death in "Of Rats and Men."
- Make Sure He's Dead: Noting that Splinter is a "resilient old rat," Super Shredder doesn't dismiss the possibility that he survived falling down the chasm, and orders his men to scour every inch of the chasm and make sure Splinter is dead. As it turns out, Splinter is indeed still alive, and the Turtles find him first.
- Makes Us Even: April saved Karai from being crushed by a tanker truck not long after Karai saved her from Tiger Claw.
- Maybe Magic, Maybe Mundane: Was Splinter hallucinating the Rat King due to a fever or was it his ghost? Splinter believes the former, but it's left ambiguous, especially as the Rat King is able to somehow wreck Splinter's broken foot.
- Morality Pet:
- Just as he's about to succumb to the Rat King, Splinter remembers Mutation Day with his sons and sees Tang Shen there.
- Karai manages to talk the Super Shredder down from beating Leo to death.
- Mythology Gag: Splinter's part of the plot echoes his "City at War" storyline from the Mirage Comics: being injured and trapped in a confined space with the Rat King and how it all ends.
- Nightmare Face: The Rat King.
- No One Could Survive That!: Shredder averts this as he notes Splinter's resilience and immediately orders his men to search the chasm. Splinter also invokes this upon rediscovering the Rat King in the chasm with him, saying he couldn't have survived the fall. He didn't. Splinter discovers his corpse and realizes it was just a fever dream brought on by his injuries.
- Nose Bleed: When using her telekinesis to lift the tanker truck to save Karai, her nose is seen to bleed just before she throws it back at Super Shredder. The first (and only) time that blood is seen in the series.
- Not Quite Dead: The Teaser shows that both Splinter and Super Shredder survived the fall last episode. Subverted with the Rat King.
- Not So Above It All: Karai takes part in the Hamato team's hand gesture.
- "Not So Different" Remark: The Rat King argues that he and Splinter are both similar, misbegotten creatures.
- Oh, Crap!: Splinter upon learning the Rat King was Dead All Along.
- One-Winged Angel: The Rat King's hallucination takes on a moving mass of rats in his ultimate attempt to corrupt Splinter into a feral rat.
- Psychic Nosebleed: When April uses her powers to push back the tanker truck Super Shredder threw, a trickle of blood leaks out of her nose. (This is the first time the show has ever had blood.)
- Raised Hand of Survival: Splinter after waking up in the pit.
- Revenge Before Reason:
- Despite everything that's happened, Super Shredder wants even more mutagen, despite Tiger Claw's urgent warnings.
- Karai manages to avert this, choosing to help the injured Leonardo rather than go after the weakened Super Shredder.
- Shockwave Clap: Super Shredder uses one to incapacitate April.
- Shout-Out: The previous episode shows Splinter and Shredder falling into a massive chasm and vanishing. This episode begins with that and shows them plummeting down the abyss as they continue to fight each other, similar to Gandalf versus the Balrog.
- Surprisingly Realistic Outcome: While Splinter surviving the long drop could be explained away by his mutant physiology, The Rat King, while being a human possessing great psychic ability over rats, is still human nonetheless and perished upon impact. Or at the very least, was horribly injured by the impact and subsequently died from his wounds.
- This Is Gonna Suck: Leonardo's reaction when he sees Super Shredder barreling down the alley towards him.
- Voice of the Legion: The Rat King ghost's One-Winged Angel form has a distorted voice.
- Wham Shot: The Rat King greeting Splinter. Again when Splinter finds the Rat King's corpse.
- Who Are You?: Hears a voice in the dark offering to help him, Splinter responds by asking this. The Rat King emerges from the shadows.
- You're Not My Father: Karai specifically tells Super Shredder he was never her father when she punches him in the heart.
