Sneakily observing the daily life of my younger sister, Rion Izumi, reveals a series of unexpectedly thrilling episodes that make your heart race. As Rion finds herself flustered by mischievous pranks from teachers and seniors, she gradually begins to give in, making your pulse quicken. Her hidden sides are unveiled one after another—animal costumes, futanari play—each more surprising than the last, leaving you gasping, "This is incredible!" Most shocking of all is the revelation that the reason she became a slightly naughty girl was due to the unexpected influence of her older brother.