In the city of Fuuto, criminals make use of USB-like devices called "Gaia Memories" to turn themselves into superpowered monsters known as "Dopants," wreaking havoc in the otherwise peaceful city. However, there are also heroes who utilize the Gaia Memories to fight these criminals, one of whom is the self-proclaimed, hard-boiled detective Shoutarou Hidari. With the help of his witty partner Philip, the two transform into Kamen Rider W—the legendary hero of Fuuto city.

After the fall of Museum—the evil organisation responsible for many crimes in Fuuto—the production and distribution of Gaia Memories has halted. However, remnants of Gaia Memories still remain within society and are sold in the black market at high prices. Thus, the two heroes from Narumi Detective Agency are yet to have time to relax. Sights of Dopants still occur and the agency receives more and more clients who claim to experience supernatural phenomena.

Regardless of the arduous nature of the task, Kamen Rider W promises that those who hurt Fuuto will inevitably count up their sins.