Sgt. Frog Episode 73: Flux Capacitor? I Hardly Flux Ca-Know It-Her!
Episode 73 of 358 of Sgt. Frog, aired August 26, 2005, 25 minutes. Available with English subtitles and dub where released.
Episode summary
Thanks to an accident with one of Kululu`s inventions, Fuyuki and the frogs time slip back to the 1980s. There, he meets his mom as a middle-schooler! Source: crunchyroll