The Sorceress sends He-Man, Man-At-Arms, Orko and Battle Cat back in time after Skeletor, who, with Tri-Klops and Fang Man, has gone back into Eternia’s past to plant an effective time bomb on the spot where Castle Grayskull now stands, that will destroy it in the present unless He-Man manages to stop it “in time”…
He-Man and the Masters of the Universe 1×06 “The Time Corridor”