He-Man and the Masters of the Universe (1983)

Season 2: Episode 40

As the heroes visit the planet Phantos, Evil-Lyn kidnaps Teela, Man-At-Arms and Orko, and takes them to Snake Mountain, where she shrinks them using a Reducto-Ray. He-Man goes to rescue them, but finds them placed in a trap which will either shrink him or kill them if he uses mere brawn to save them. So he realises he must use his brain to solve the problem- and since Teela has been complaining that Adam is lacking in brain power, he decides to do the job as Adam.