At night, a woman is attacked and killed in a gruesome way. Soon afterwards, a young couple in a car is targeted as well, but when they're about to be attacked, police comes and try to stop the killer. However, they fail, and the killer, a monstruous creature escapes. At the same time, a young man (Shin Kazamatsuri) wakes up, and is soon comforted by a nurse, who is also his girlfriend, Ai Asuka.
Then, a bunch of cars enter a scientific research center, the Institute of Super Science (ISS) and a bunch of people are greeted by the ISS's head, Iwao Himuro, who makes them visit their research laboratories. In one of them, we see the young man, his girlfriend and two researchers (Daimon Kazamatsuri, Shin's father, and Giichi Onizuka). Himuro explains that the aim of the research is to make the human body stronger by increasing regenerating abilities, to help curing diseases like AIDS and cancer. Shin seems to be the experiment subject. At the lab, we see the two researchers argue, Onizuka wanting to go at a further stage of their experiment when Shin's dad being more cautious, especially since the test subject is his son, even if Shin doesn't mind being so. Soon afterwards, we see Shin and Ai do some exercise in a sports center, and there they meet Yuki, Shin's friend, who reminds him of his dream as a motorcycle racer, which he gave up to help his father for his research. However, then, we see Shin and Ai have some sweet time together, as they're spotted by a woman with sunglasses.
The next evening, Shin is jogging when he sees Onizuka going alone in the night. He follow him to an abandoned building, only to see him doing crazy experiments on himself in a lab made there , and when Onizuka feels pain, Shin feels pain at the same time. Then, the lab is attacked by a bunch of people with firearms, led by the woman with sunglasses, who shoot everywhere in the lab, especially targeting Onizuka, who tries to avoid the bullets. Shin tries to avoid the shootings, but soon loses consciousness while a sudden change happens on Onizuka, and the armed people are killed in a gruesome way. When Shin wakes up, he sees the bodies of the killed agents, and he believes he's the one responsible for the massacre. The next day, he meets Onizuka, because he needs to know more about the research, threatening him to talk about the events of the night. Onizuka explains that his research focused on the strange powers seen in different animals, especially insects, and that he's fascinated by insects, especially grasshoppers, which he thinks are the creature of the future, more than humans. He's especially fascinated by their telepathic powers. He then tells Shin, and his father that the research is gonna go on, even if they don't want to. Shin comes back to the building, and there meets the woman with sunglasses, Sarah, who tells him that her aim is to destroy the syndicate who pays for the ISS, and that they're looking for the same thing. But as they're about to talk more, Sarah noticed that they're targeted by a sniper (already seen before as Himuro's assistant). Sarah saves Shin, and both her and the sniper shoot at each other. At the end, Shin manages to escape, while Sarah seems to kill the sniper, after shooting him and making him fall from the half destroyed building.However, it is revealed later that he survived.
Shin goes back to the sports center and the pool, and there, meets Ai and Yuki, and tells him of his fear and horror of being a killer, , even wanting his friends to leave him because he's dangerous. However, Ai stays with him and tries to comfort him by swimming in the pool with him.
The show then shows Onizuka locked in a straightjacket, and kept prisoner by Imuro, who mocks his passion for insects, and also mocks science as well. Himuro then meets Shin's dad to tell him that Onizuka has quit, and that he must go on with the research, while at the same time telling mysterious people by phone that he's gonna send an interesting sample this night. Himiro clearly shows Shin's dad he has no other option and keeps him prisoner. Shin then questions why noone is concerned by the fact someone's missing.
The next night, Shin investigates the science center when he begins to feel a painful feeling, like if someone was calling him. He visits Onizuka's lab, full of cages with grasshoppers. At the same time, security guard are closing a truck which seems to contain a dangerous creature, and then a convoy with the truck and a car leave the research center. Shin follows the convoy with his motorbike, because he seems to be able to communicate with the trapped creature. The convoy is then ambushed by a big truck, which makes the convoy's truck lose control and falling. At the same time, the big truck opens, and a bunch of gunmen, led by Sarah, start to shoot at the guards., with deadly results. However, Himuro's assistant escapes, while Sarah and her men approach ISS's truck, and decide to make it explode with a bazooka. In ISS's truck, a creature tries to get out. Shin tries to stop Sarah and her men to destroy the truck, but in vain. Flames appear, and an insect like monster is burning in the flames, and eventually dies. The monster is revealed to be a transformed Onizuka. At the same time, Shin begins to experience incredible pain, and ends up transforming himslef into a big scary green grasshopper-like monster. As Sarah witnesses the transformation and is hesitating about shooting the monster, Himuro's assistant comes, and tries to kill the young woman, who's saved by Shin in monster form. Himuro's assistant himself turns into a monstruous creature, and fights Shin. At the end, the fight ends without any winner, and Shin runs away, while Sarah is unable to shoot him with his bazooka. Shin (in monster form) then meets Ai who apologizes to him, and he takes her in his arms (still while being a monster). The next day, Sarah talks in English to her boss and tries to explain that despite being a monster, Shin seems to have kept his human heart, and tries to convince her boss not to kill him. However, she fails, and is ordered to kill him. At the same time, Ai explains everything to Shin, that Sarah is a CIA agent, and that the real aim of the syndicate and ISS is to use the research on regenerating powers to create super strong soldiers, which would be sold and bring money to the syndicate. Onizuka did crazy research on grasshoppers because of his beliefs, and ISS let him do so, since it could help for their goals. Indeed, Onizuka managed to get to the next step of the research, and has changed himself and Shin into true monstruous and very powerful human-grasshoppers hybrids. It was for that reason that Shin and Onizuka were able to communicate through telepathy At the same time, Himuro explain the same to Shin's dad, explaining how Shin is a precious sample now. Ai geos back to ISS to confron her boss, but is soon captured as well, and then, learns that she's pregnant with Shin's child, who is gonna be a human- grasshopper creature as well.
Ai soon experiences pain from her pregnant state, and Shin's dad demands help from Himuro. Himuro opens the door of the room where Ai adn Shin's dad are locked, but at the same time, a yellow light appears (which shows the unborn child's powers), and incapacitates Himuro and his men, and the two prisoners run away to escape.. Shin receives the telepathic cry of the creature (even if he ignores its existence so far), and runs towards the center. Ai and Shin's dad try to get out of the center, but every door is closing in front of them, and they soon end up trapped. However, Shin tears up the door and comes to help. They're soon attacked by both the ISS's security men, and at the same time, Sarah's men who are attacking ISS. Shin, his dad and Ai try to escape only to confront Himuro, who, with nothing to lose, decides to kill them. He tries to kill Shin and mortally wounds Ai who tried to protect Shin. Ai dies, her last words being to ask Shin to protect her child, whatever appearance he has. Furious and full of grief, Shin turns into his monster appearance and kills the frightened Himuro in a very bloody way. Soon afterwards, Hiromu's assistant comes in monster form, and the two creatures sart a very violent fight to the death, with Shin eventually severing his opponent's head and spine from the rest of its body. The head explodes, and Shin and his dad escape with Ai's body. Outside, they meet Sarah and her men. Sarah explains that she has no choice but kill Shin, but as she's about to shoot, an helicopter comes (likely from the Syndicate), and Sarah and her men are shot and bombed from above. Shin's dad is captured in a net sent by the helicopter and Shin himslef is captured, while trying to save his dad. However, Sarah is able to shoot the helicopter with her bazooka, destroying it and the buiding below (and likely, Shin's dad as well). The story ends with Shin carrying his girlfriend's dead body, his only hope being his future child.
Shin Kamen Rider prologue is a very violent movie. Almost every character in the movie die, and in a lot of cases, in very gruesome and bloody ways. The movie ends in a pretty depressing way, the hero having lost almost everything in his life, his father, his lover, and he's even not human anymore. His only ray of hope is his future child.
While the movie is very violent and that the special element of that show is how violent and dark it is, since the violence isn't that different from early toku violence, notably Showa era Rider shows. However, it's true that the violence is shown much more crudely than in Rider TV shows, even showing the hero in monster form violently killing a defenseless man (as despicable as the man was)
The storyline uses elements already seen in previous rider shows : Kamen Rider Black has shown the hero hero being subject of experiment by his father, with the latter realizing too late the true meaning of the organization he's been involved with, and paying the ultimate price for it. The concept of the girlfriend involved in a way has been done in Kamen Rider X.
However, what makes that movie so special is the way the story is told. Shin Kamen Rider prologue is basically an extended first episode of a Showa Rider show Usually, in those "first episodes", the way the protagonist becomes a "remodeled man" is detailed. Here, instead of being a 25 minutes long, it's a 1 hour and a half movie. Thus, the concept can be explored in much more depth than in a 25 minutes episode, and more elements can be introduced, like the crazy scientist and the CIA subplot.
There is another important element that makes the movie even more stand out its storyline : instead of being a crazy cult which members are mostly monsters or cyborg like creatures, here, the antagonists are all human with the Big Bad being a corrupted executive, who works for a creepy syndicate.
Instead of showing us villain with over the top outfits and animal like monsters, the real villains were normal looking people wearing ordinary clothes. It gave the movie a more realistic aspect and highlighted much more than in previous Rider shows the evil in humans.
Another interesting element of the story is that every character is tainted : even the most sympathetic characters have been in a way corrupted: the hero's dad and girlfriend, who were accomplice of the horrible stuff done to him. Even the hero himself was at first a willing participant to the crazy experiments he was undergoing, and at the end, kills a human with his own ends, despite the latter being unable to fight back (of course, said human was the movie's main antagonist, who has killed the hero's girlfriend just before, and it was difficult not to cheer Shin when he killed him) . The people who fight the syndicate (the CIA agents, led by Sarah) show also a ruthless side, having no problem killing people involved in ISS.
While the movie was a new approch to the Kamen Rider mythos, it's interesting to see that the involvement of humans as villains instead of monsters seems to come from the Metal Hero shows from that era, like Winspector, which had in a lot of cases, ordinary criminals as antagonists instead of the usual Monsters of the Week. Indeed, the more realistic tone is pretty similar to the one appearing in the Metal Hero shows of that era.
It's worthy to notice that one of the writers of the show is Junichi Miyashita, who has worked on Winspector and he would use a lot of the themes used in that movie in a Metal Hero show he would be head writer of : Janperson. Indeed, the Institute of Super Science would find its spiritual successor in the Super Science Network (SS-N) in Janperson, which is one of its villain group. Like the ISS, SS-N is doing experiments to make powerful mutant creatures, and is not above using humans to make them. Moreover, the concept of the corrupt executive antagonist would be used in Janperson , with the character of Ryusaburo Tatewaki, leader of the Tatewaki conzern, who also would use violence and deception to reach his aims.
Lastly, Shin Kamen Rider movie prologue wouls also have an influence on Heisei Rider series. In the movie, Sarah is seen to have trouble dealing with Shin, because he's a monster. In Agito, the conflict between official forces and the too powerful Riders would be also a big theme. Heisei Rider series would also use corrupt companies as antagonists, like the Smart Brain in Faiz and the Foundation X in the W/ Fourze universe. Lastly, in both W and Fourze, the antagonists are corrupted people closely associated with the heroes (especially Fourze).