After settling into his new Tokyo apartment as an art student, Tatsuhiko Kido notices a small opening in the wall and soon encounters his neighbor Ikuno Emiru. He visits her unit intending only to mention the discovery, yet an unexpected tumble leaves them in an awkward position that she quickly photographs. To avoid any fallout from the image, he agrees to her unusual proposal of mutual observation. Their shared secret begins to shape his daily routine and personal boundaries from that day forward.