Kouji Segawa is a reporter whose little friend, Kana, is carried off by the monstrous children of the alien Fog Mother to feed to her newly-hatched offspring. Kouji is given the mystical "J Power" to become Kamen Rider J and defeat Fog Mother before she can destroy mankind.
J would appear in several later Kamen Rider movies, with the cameos being more minor as the years go on.
- Kamen Rider World (1994) - 3-D Movie crossover with Kamen Rider ZO
- Kamen Rider Decade: All Riders vs. Dai-Shocker (2009)
- Kamen Rider Decade: The Last Story (2009)
- OOO, Den-O, All Riders: Let's Go Kamen Riders (2011)
- Kamen Rider × Super Sentai: Super Hero Taisen (2012)
- Heisei Rider Vs Showa Rider Kamen Rider Wars Featuring Super Sentai (2014)
- Super Hero Taisen GP Kamen Rider 3 (2015)
Kamen Rider J provides examples of:
- Badass Biker: Kamen Rider, duh!
- Crossover: With Kamen Rider ZO in the 8-minute short Kamen Rider World. Guest-starring monsters from several Kamen Rider installments and Shadow Moon.
- Eldritch Abomination: The Fog Mother is a giantic cybernetic doomsday monster whose sole purpose for existing is to feed civilizations to her monster offspring. And that's even mentioning her true form.
- Flanderization: In later appearances J's ability to grow is the only thing about him that factors in at all.
- Good Hurts Evil: The Fog Mother feels intense pain when J is first given his powers.
- Green Aesop: It appears here and there, though the Fog Mother's plan doesn't revolve around it.
- Make My Monster Grow / Super Mode: A heroic example where J does this to himself to defeat the giant Fog Mother.
- Mama Bear: Fog Mother is actually quite ticked off when J kills her babies and tries to take him down with her as she's dying because of it.
- Mighty Glacier: J in Jumbo Formation. He trades off his speed for increased strength, as described with the entry for Square-Cube Law below.
- Nature Spirit: A group of weird plant people who bring Kouji back to life as Kamen Rider J and task him with destroying the Fog Mother and her evil spawn, they state they're spirits of the Earth.
- Phlebotinum Killed the Dinosaurs: The Fog Mother in this case.
- Shout-Out: Kamen Rider J's name is a reference to the Japanese Pro Soccer League, better known as "J-League", which was founded a couple of years before the movie premiered.
- Square-Cube Law: When J assumes his Jumbo Formation, not only does he move much slower than before, but the ground actually does collapse under his weight. Essentially, he becomes a Mighty Glacier with all the extra weight that comes with it.
- Taking You with Me: After being mortally wounded by J, Fog Mother tries to take J with her.
- Technicolor Death: Oddly averted for the first three monsters, who simply die...but more than made up for with Fog Mother who goes out in multiple giant explosions.
- Terrible Trio: Agito, Zuu and Garai.
- Touch of the Monster: Kana passes out the instant she's carried off by Zuu, and spends nearly the entire film unconscious.
- The Worf Effect: Several later appearances of J have him in giant form all the time - only to lose to something even more powerful. In Kamen Rider Taisen he's downed in the Cold Open!
- Would Hurt a Child: Kana is captured as part of a ritual the Fog Mother intends to carry out.