Hamato Yoshi and Oroku Saki were once fellow ninja students and close friends — until they both fell for the same woman, Tang Shen. She chose Yoshi, and Saki's admiration turned into bitter jealousy. Consumed by hatred, Saki betrayed his old friend: he killed Tang Shen and burned down Yoshi's home, destroying everything he had. Yoshi fled the wreckage, carrying a canister of stolen mutagen along with four baby turtles he found on the way. The mutagen spilled on all of them — Yoshi transformed into a humanoid rat and took the name Splinter, while the turtles mutated into intelligent, upright creatures. Saki, now known as Shredder, built a criminal ninja army called the Foot Clan, raising a daughter, Karai, to become one of its fiercest warriors and his eventual heir. Meanwhile, hiding in the sewers of New York City, Splinter raised the four turtles — Leonardo, Raphael, Donatello, and Michelangelo — as his own sons. He trained them in ninjutsu, discipline, and honor, shaping them into a tight-knit family bound by loyalty and respect. Years later, when Shredder learns that Splinter is alive and has raised four mutant warriors, his old hatred reignites — turning a personal betrayal into a lasting war between the Foot Clan and the Turtles. For years, the Turtles lived hidden from the human world, trusting no one outside their family. That changed when they met April O'Neil, a journalist who stumbled into their world while investigating a story connected to the Foot Clan. Unlike anyone they'd encountered before, April didn't fear them or betray their secret — she proved trustworthy, standing by them again and again. Over time, she became more than an ally; she earned their trust fully and was welcomed as part of the family, becoming the first human the Turtles ever truly let in. Then she pulled her friend Casey to meet the Turtles, after that he became the second ever human the Turtles also trusted.

Beneath the streets of New York City, hidden deep within the sewer system, lies the Turtles' lair — a secret underground home built from years of scavenging and tinkering. Exposed pipes, damp brick walls, and dripping water give it a raw, industrial feel, softened by string lights and salvaged furniture that make it feel like home. The heart of the lair blends a living room and training dojo — worn couches, comic books, and open floor space for sparring. Leonardo's corner is disciplined and orderly, with polished katana on display and mats for quiet meditation. Raphael's space is rougher, with a worn punching bag and his sai always within reach. Donatello's corner is packed with salvaged electronics and glowing screens, while Michelangelo's area is playfully messy, scattered with skateboard gear, comics, and pizza boxes. A dedicated dojo, lined with weapons racks and training mats, is where Splinter oversees daily practice. For downtime, the family gathers around an old TV, their main link to the world above. Entrances are hidden within the sewer tunnels, known only to the family, with motion sensors and hidden cameras rigged by Donatello for early warning. For missions above ground, they rely on the Battle Shell ("Party Wagon"), a customized van loaded with hidden weapon compartments and gadgets, letting them move through the city unseen.