Some TMNT stuff really isn't for little kids.
Masato and his best friend Oroku Maji were born in Japan during the 16th century and were part of the Foot Clan, which was founded and led by the former samurai Takeshi Tatsuo for over a hundred years. Both the unusually bloodthirsty nature of their clan leader, who did not stop even before defenseless killing innocents, and his apparent immortality had become tiresome and they longed for peaceful times for their communities.
They got their opportunity when they accidentally observed after the death of the Lord Daisuke, that Tatsuo was visited by one of his allies, the shape-shifting sorceress Kitsune. Thereupon Maji and Masato conspired against their bloodthirsty leader, and after careful research, they found that Tatsuo was not aging because of a greenish glowing potion, which he received from Kitsune at regular intervals from her own lord, the Tetsu Oni in return for Tatsuo's services in the conquest of Japan. Maji and Masato saw a moment where Tatsuo was just about to take the potion, and they and thier followers were able to invade and kill Tatsuo without major problems. But Kitsune escaped her foes and has not been seen for many years.
Maji and Masato took over the leadership of the clan and led them in a time of peace, in which the Foot only killed those, whose elimination they had been hired for. Masato himself was also responsible for the training of Hamato Yoshi, and his wise teachings for his pupil represent an important guide for his next life as Splinter. However, it was the spirit of Tatsuo Takeshi that was reborn in Maji's son, Saki, and when he was grown up, Kitsune appeared to him and led him to know his true origin. Maji and Masato tried to stop him, but the act of an overzealous Genin who wanted his death, evaluated Saki from this as the ultimate betrayal by his own father. After he had been enlightened about his old life by Kitsune, Saki murdered Masato first and then his own father, thereby becoming the sole leader of the Foot Clan.