Call-Back: Murakami returns after being absent since "I, Monster".
Chekhov's Classroom: April observes Splinter teaching Raph and Mikey how a captured ninja should do anything to break a foe's concentration in order to escape. Later during the fight, April recalls what he said before taking advantage of Karai's hesitation.
Cool Bike: Karai owns a sleek red-and-black motorcycle. It appeared for a brief moment in the previous episode, but gets a lot more screen time here. It bares more than a passing resemblance to a Kawasaki Ninja.
Curb-Stomp Battle: She managed to land one lucky hit and take advantage of a momentary distraction to win at the end, but April was no match for Karai.
Did I Say That Out Loud: After the turtles have gotten back to the Lair and Donnie is frantically trying to contact April on his T-Phone.
Donnie: Come on, April! Come on, April!
April: Hey Donnie!
Donnie: April! You're okay! You hear that, guys? My sweet princess is alive! [Beat] Did I mute that?
April: Let's agree that you did.
Foreshadowing: April is unharmed by the pure Kraang-water that a moment before had completely dissolved a pizza slice.
Green-Eyed Monster: Karai demands to know what makes April so special while she's pounding her into the dirt.
Hamster-Wheel Power: The Turtle Sub is equipped with jets for quick action, but to charge them up and for stealth it also runs on three bicycles that move the flippers. Leo, Raph, and Mikey are the power source for those.
Heroic Bystander: Murakami dumps a bowl of soup on Karai and stalls her to help April escape, all while pretending to be a blind klutz.
Heroic Resolve: No matter how many times Karai beats her senseless or throws her down, April just keeps getting right back up try again.
When April calls the turtles for help against Karai, Don wants to abort the mission and help her, but Raph points out that foiling the Kraang's water poisoning plans are more important otherwise they'll be endangering everyone in New York including April.
In Space Heroes, the Captain states that if they keep bailing out their trainees, they'll never learn how to take care of themselves. This falls under Sink-or-Swim Mentor given the circumstances of those trainees.
Made of Iron: The Kraang creature survives the explosion of the base with no ill effects.
Reality Ensues: In addition to April's training as a kunoichi averting Instant Expert, her fight with Karai makes no attempt to dodge the hard truth that someone who's been training for years and has been in actual fights is going to beat someone who has never been in a serious fight and has only been training for, at most, a few months.
Staircase Tumble: Karai doesn't so much fall down the subway stairs as get bodily thrown down them by April.
Stock Ness Monster: The Kraang sea creature guarding the base bears a great resemblance to many illustrations of Nessie and plesiosaurs in general.
Tempting Fate: While in the Kraang's underwater lab, Leo tells the others to be quiet. Don points out there's no need for Leo to say that everytime since they're ninjas and they know how to be quiet. Cue Don's T-Phone ringing.
You Can't Go Home Again: With both the Kraang and the Shredder out to capture her, Splinter and the Turtles decide that April will have to stay in the sewers with them for her own safety, at least until the crisis is resolved.