Kamen Rider ZO is one of the stand-alone Kamen Riders, made after the success of the direct-to-video Shin Kamen Rider: Prologue (which did things differently, especially having the hero mutate into his rider form, as that film was basically a more realistic take on the character). For ZO, the dark tone is there, but it goes back to the basics.
In this film, Masaru Aso awakens from a coma after he receives a call telepathically to protect Hiroshi Mochizuki, the son of the doctor who did an experiment on him, causing him to transform into the grasshopper-themed Kamen Rider ZO. Masaru must defend Hiroshi from the monster Doras, sent by the Neo Organism, who has Dr. Mochizuki and plans to capture Hiroshi as a way to force the doctor to complete its evolution into its perfect form so it can destroy the human race, which it finds imperfect.
This movie was a return to the roots for Kamen Rider. Despite not having a henshin belt, Masaru does transform into a grasshopper-themed suit, with the red circle being built into the suit due to the hero not utilizing a henshin belt like past Kamen Riders, but is done in a way that it could be seen as a homage to them. Also, the special effects and monster costumes are amazing (with Doras getting the most, as his formation at the start uses what looks like computer animation, and some techniques, like fading, being used for his ability to repair itself). Also, the fight sequences are well done (Kamen Rider ZO battling Kumo Woman, a spider-like monster, being impressive, as the creature is stop-motion animated in some scenes, but an animatronic in close-up shots). This was planned to be continued in a TV series, but creative issues and other problems prevented it. However, scenes from this movie were used for Saban's Masked Rider (as the three monsters were used), and the movie did come to America as an FMV game for the Sega CD (which had one of the worst English voice dubs I have ever heard, but would include extra footage not seen in the film). Thankfully, Toei's YouTube channel, Toei Tokusatsu World Official, has it in full (complete with English subs, which is what I watched for this review), so if you are a Kamen Rider fan, this is one to check out.