In 2009, following a wave of global terrorist attacks, the Japanese government established a black-ops anti-terrorist unit known as Shade. However, the organization exceeded its mandate by conducting inhumane experiments on kidnapped civilians to create a force of mind-controlled super-soldiers. When these human rights violations were uncovered, Shade was officially disbanded and its leader, Seizan Tokugawa, was imprisoned. The remaining operatives went rogue, continuing their activities as a terrorist organization determined to free their master.

The story centers on a young man named Goro, a sommelier who disappeared three years prior under mysterious circumstances. He was, in fact, a victim of Shade's experiments, having been abducted, brainwashed, and physically remodeled into a soldier designated as Operative Number 5. The narrative begins in earnest when Shade's remnants storm the TV Asahi building during a live broadcast, taking everyone hostage to demand the release of Tokugawa. Among the hostages is Eri Hinata, a television host and Goro’s former fiancée, who is stunned to recognize the amnesiac operative as the man who once promised to marry her.

During the standoff, the brainwashed Goro confronts Eri. Though he shows no sign of recognition, he impulsively drinks a glass of wine she had left on the set, the very vintage he had served her during his marriage proposal three years prior. The taste acts as a powerful catalyst, shattering Shade's conditioning and restoring his original memories and personality. Overcome with emotion and a renewed sense of justice, he turns on his captors. Using a unique transformation device composed of a wine bottle and a corkscrew-shaped belt, Goro transforms into Kamen Rider G, the solitary warrior of love, and fights to protect Eri and the innocent people in the building.

The conflict escalates when the Shade leader on site, Daido Oda, and his subordinates reveal their true natures as the Phylloxera Worm and various other Worm-type monsters, using repurposed enemy designs from Kamen Rider Kabuto. After a fierce battle, Oda abducts Eri and escapes by helicopter. Goro gives chase on his motorcycle, leaping into the air to rescue her and confront the villain. Overwhelmed by the monster’s power, Goro is on the verge of defeat when he is visited by the dimensional-traveling Kamen Rider Decade. Decade, alongside the spirits of the nine primary Heisei-era Kamen Riders, encourages Goro, reminding him that he is the only one who can protect his world. Reinvigorated, Goro destroys the Phylloxera Worm with a final, decisive attack. Though he is reunited with Eri, Goro understands his duty as a Kamen Rider. He vows to save the world from the remnants of Shade, running off into the unknown and leaving Eri behind, his identity and mission now a secret he must carry alone.