In the ecologically-minded city of Futo (Futo, 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, his relation to the Sonozaki Family, and their Museum organization are revealed.