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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 (園咲家, Sonozaki Ke^?) sell mysterious devices resembling USB flash drives called Gaia Memories to criminals who use them to become monsters called Dopants, committing crimes with the police force powerless to stop them. The hardboiled (or rather half-boiled) detective Shotaro Hidari works with the mysterious Philip who possesses the true Gaia Memory. 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

source : wikipedia