Some TMNT stuff really isn't for little kids.
T'Pau was a Yeti mystic living in Tibet. During one of Splinter's meditations, T'Pau visited him in a dream walk and told him of a threat that was coming to Earth. Back in the physical plane, Splinter conscripted his sons to help his new friend.
Arriving in Tibet, they are just in time to see T'Pau being captured by an alien. T'Pau, however, says he means no harm, and that he only wants him to to return with him on his ship. The Turtles attack anyway, and Donatello, Raphael, and Leonardo, as well as Splinter, are blasted away by the alien, before Michelangelo and knocks the foe silly. Although disoriented, the alien sees this as his chance to nab Michelangelo and T'Pau as well.
T'Pau and Michelangelo awaken on the alien's ship, and Michelangelo uses a lockpick he had hidden in his belt buckle to free them. The two of them knock out a couple of guards, and meet their friends safe and sound on the ship's bridge.
Splinter introduces the alien as "Boss Salvage", and Salvage apologizes and it's taken time to learn their language. He describes himself as a "Keeper" - a collector of rare beings from dying worlds. Since the environment of Earth is being depleted rapidly, he has begun gathering together specimens of the "last of their kind"s - including Nessie, Bigfoot, and Kingfish the merman. T'Pau, however, convinces Boss Salvage that the Earth is not irredeemable, and indeed, there are people that have begun turning the tide already. Boss Salvage relents, and T'Pau sends his friends home, but not before giving Michelangelo a special speech of gratitude.