He-Man and the Masters of the Universe Season 1 Episode 63 The Once and Future Duke

While enjoying a lazy break fishing, Adam and Cringer see a young boy about to be eaten by a man-eating plant, and come to the rescue as He-Man and Battle Cat. Afterwards, Adam and Teela recognise the boy as their childhood friend David of Athra - only whereas they've grown, David looks the same as he did then! The Sorceress tells Adam that David's evil uncle, magician Count Marzo, cast a spell to transform him back to a child and wipe his memory. The only hope of restoring David to his proper self is to find the Ring of Remembrance, which is located in the Lake of the Lost. But, while He-Man, Teela and Battle Cat search for it, Count Marzo soon captures David back, and intends to use his magic powers to ensure He-Man and co. can never stop him...