Motorcyclist Takeshi Hongo is brutally kidnapped by the nefarious Sustainable Happiness Organization with Computational Knowledge Embedded Remodeling (SHOCKER), a malevolent group with a sinister agenda of converting humans into brainwashed, synthetic animal hybrids, referred to as Augments, or Augs. Hongo undergoes a chilling transformation into a creature known as Grasshopper-Aug 01; however, in a twist of fate, he is liberated by Ruriko Midorikawa, a former SHOCKER operative, before the organization can complete his brainwashing. Soon after, SHOCKER dispatches one of its fierce agents, Spider-Aug, to reclaim them, capturing Ruriko momentarily, but Hongo intervenes in his augmented form, showcasing a blend of human resolve and synthetic power.
With tension high, Hongo and Ruriko seek refuge in a safe house, where they meet Ruriko's father, Dr. Hiroshi Midorikawa. In a pivotal conversation, Dr. Midorikawa discloses that Hongo is his ultimate creation – an Augment destined to thwart SHOCKER's ambitions. He informs Hongo that he was selected due to his desire for strength after a traumatic incident where his father, a dedicated policeman, was killed by a criminal during an arrest attempt. Additionally, Dr. Midorikawa explains that a life force, known as prana, grants Hongo superhuman abilities, enabling him to battle adversaries with remarkable strength and agility. Tragically, their conversation is interrupted by the sinister Spider-Aug, who kills Dr. Midorikawa and kidnaps Ruriko again. Hongo’s determination surges as he resolves to use his newfound powers for justice, shedding the title of Grasshopper-Aug in favor of becoming Kamen Rider. Although Ruriko remains skeptical of his transformation, she reluctantly begins to trust his intentions.
As the story unfolds, two mysterious government agents approach Hongo and Ruriko with a compelling proposition: forming an alliance to dismantle SHOCKER in exchange for critical intelligence on the organization’s operations. Ruriko unveils SHOCKER's origins, revealing that it was initially conceived by a billionaire who aimed to usher humanity toward happiness. Tragically, after the billionaire's suicide, SHOCKER fell under the control of an artificial intelligence, which distorted its creator's vision, misconceiving that humanity's salvation required its domination.
Following the government’s directives, Hongo and Ruriko engage in fierce battles against several Augs, including Ruriko's friend Hiromi, who has been transformed into Wasp-Aug. When Hongo cannot bring himself to end her life, tragedy strikes as Wasp-Aug is killed by one of the unnamed agents utilizing the venom from Scorpion-Aug. Later, in a heart-wrenching twist, Ruriko’s brother, Ichiro, is awakened as Butterfly-Aug before she can complete a critical code designed to suppress him. Fueled by rage over their mother’s untimely murder, Ichiro unleashes chaos, sending many government agents to a horrifying dimension known as the Habitat Realm.
Hongo and Ruriko rally to confront Ichiro but find themselves overpowered, leading Hongo to a fateful encounter with another SHOCKER recruit, Hayato Ichimonji, now transformed into Grasshopper-Aug 02. While Hongo barely escapes with Ruriko, he suffers grievous injuries at Ichimonji’s hands. Remarkably, Ruriko manages to liberate Ichimonji from SHOCKER's clutches, only for her life to be abruptly cut short at the hands of the K.K.-Aug, who is subsequently vanquished by Ichimonji in a tragic turn of events.
In a moving show of ingenuity, Hongo accesses Ruriko's last message through his helmet, where she reveals that her strength was no match for Ichiro’s overwhelming prana. She confides that she has implanted a code within his helmet capable of neutralizing Ichiro and expresses how her companionship with Hongo brought her genuine joy. Overwhelmed with grief over her demise, Hongo attempts to persuade Ichimonji to unite forces against SHOCKER while soliciting a favor from the two unnamed government agents in a desperate bid to face Ichiro.
As Hongo is ambushed by a legion of Grasshopper-Augs, Ichimonji, now embracing the mantle of Kamen Rider No. 2, comes to his rescue, and together, they coalesce their efforts against the relentless foes. Ultimately, they confront Ichiro, who unveils his true form as Kamen Rider No. 0, displaying formidable might even as Hongo and Ichimonji strive to sever his source of prana. In an emotional finale, they manage to remove Ichiro’s helmet, enabling him to connect with Ruriko’s spirit, leading to a poignant reconciliation. This battle exhausts their prana reserves, resulting in their untimely demise.
In the aftermath, the two government agents honor Hongo's wish by entrusting Ichimonji with Hongo's helmet while securing Ruriko's soul, emphasizing that Hongo envisioned Ichimonji as the bearer of the Kamen Rider legacy to combat SHOCKER. With newfound purpose, Ichimonji joins forces with the agents, Tachibana and Taki, embarking on a mission to uphold Hongo and Ruriko’s ideals. Clad in a revamped suit and fortified helmet, Ichimonji shares a moment with Hongo’s essence within the helmet, setting forth into the horizon as Kamen Rider No. 2+1, ready to confront the darkness ahead.
