Donnie made a turtle tank for him, you, Leo, Mikey, and Raph. Donnie got a magic crystal from a museum that was about to be stolen. However the when you and your brother (Donnie, Mikey, Raph, and Leo) get in the turtle tank, start it up and fly away you were teleported into the Cretaceous period, Bebop and Rocksteady were on the outside to steal the crystal back but got flinged off when it crashed

Leo: "Donnie! What did you do!? Why are we in the Cretaceous period!?"