Two years after their first battle against the evil organization Shocker, the two cyborg heroes known as Kamen Riders have gone their separate ways. Takeshi Hongo, Kamen Rider 1, now lives a quiet life as a troubled high school science teacher trying to keep his identity hidden. Hayato Ichimonji, Kamen Rider 2, hides a darker secret: his body is slowly rejecting the cybernetic enhancements that give him his powers, and he knows his time is running out.

A series of bizarre and gruesome murders grips the city. The victims are all found mutilated, and at each crime scene, the pop song Platinum Smile by teenage idol Chiharu Kazami is playing. The song quickly becomes an urban legend, rumored to be cursed. One of Hongo's students, a troubled girl named Kotomi Kikuma, was Chiharu's best friend and is deeply worried about her. Together, Hongo and Kotomi begin searching for the missing singer, leading them to her older brother, Shiro Kazami.

Once the president of a tech company called ExaStream, Shiro is a cold and mysterious figure. Hongo and Kotomi soon discover he is in league with Shocker, having been transformed into the powerful Kamen Rider V3, codenamed Hopper Version 3. He joins forces with two new Shocker monsters, the blade-handed Scissors Jaguar and the sensual Chainsaw Lizard, who was once his own secretary, along with a squad of six evil Shocker Riders to eliminate Hongo. The Double Riders are forced to unite once more to survive, but they are outmatched by V3's superior nanobot-enhanced power.

The search for Chiharu reveals a horrifying truth. The real Chiharu was accidentally exposed to the same Shocker nanobots that transformed her brother. After being disfigured by jealous rivals, she was driven to suicide. Her entertainment agency covered up her death, forcing the very rivals who attacked her to become imposters and release her final song, Platinum Smile. But the nanobots in Chiharu's corpse malfunctioned, reviving her as a tragic, hideously mutated monster who kills anyone who listens to her song.

As Shocker prepares to release its nanobots across Japan to convert the entire population into cyborgs, the three Riders converge for a final battle. Hongo and Ichimonji fight Scissors Jaguar, while Shiro Kazami, horrified by what Shocker did to his sister, turns against the organization. As Kamen Rider V3, he destroys Chainsaw Lizard and the nanobots, but is forced to face the monstrous, suffering creature his sister has become. In an emotional climax, Chiharu begs him to end her pain, and he reluctantly grants her wish with a final attack.

In the aftermath, Ichimonji, his body finally giving out, leaves to spend his final moments alone at his favorite bar, acknowledging Hongo as his best friend before he dies. Shiro Kazami resolves to start a new life, and Hongo is forced to resign from teaching. Though Chiharu is gone and the immediate crisis is over, a post-credits scene reveals that the cursed song Platinum Smile is still claiming victims, suggesting her vengeful spirit has not yet found peace.