This paper scrutinizes the confluence of idol and racing industries in Uma Musume Pretty Derby, a Japanese multimedia franchise centering on "horse girls," anthropomorphized characters based on racehorses. I study how the franchise elicits audience's emotional and financial investment by combining features of idol performance and racehorse breeding in an "affective economy," while highlighting its strategy to reconcile the contradictory concepts of purity through a process I call "purification." This paper uses the franchise to critically engage the imperial and eugenic legacies in the racing and idol industries of Japan, and demonstrates the complicated entanglement between science, empire, and entertainment.