隣の蟹と恋をした

Ayaka Sasaki, an ordinary, good-natured girl, has a boyfriend, Misaki, whom she met at a blind date mixer. She stays over at his place on weekends, and she's happy with the arrangement, even though they've had some disagreements lately. That is, until a handsome guy named Kanie moves in next door and tells Ayaka that she is "being used"? (Source: Shuueisha, translated)