In the ecologically-minded city of Futo (風都 Fūto?, the “Windy City”), there are many windmills and the people live in harmony. However, the Sonozaki Family sell mysterious devices resembling USB flash drives called Gaia Memories to criminals and other interested parties who use them to become monsters called Dopants, committing crimes with the police force powerless to stop them. To make matters worse, the Gaia Memories carry a dangerous toxin that cause their users to go insane to the point where they could die from the life-threatening devices. After the death of his boss, the self-proclaimed hardboiled (actually half-boiled) detective Shotaro Hidari works with the mysterious Philip, who possesses the true Gaia Memory, to investigate crimes which involve Dopants. Using their own Gaia Memories, Shotaro and Philip use the Double Driver belts to transform and combine into Kamen Rider Double to fight the Dopant menace and keep Futo safe. While joined in their fight by investigator Ryu Terui, who transforms into Kamen Rider Accel, the mystery of Philip’s past and his relation to the Sonozaki Family and their Museum organization are revealed.
所謂的「Dopant」實際上是人類使用「蓋亞記憶體（Gaia Memory）」變身的模樣。這些Gaia Memory經由一家企業正在檯面下流通著。操縱這些Gaia Memory流通的，則是園咲家族成立的企業──家庭的每個人在風都各擔任著不同的職務，但知道他們真面目的人少之又少。