Synopsis so far: Guy wants to break up with his girlfriend because he finds (thinks, or is made think) that he can't really make her happy, and sometimes can't really understand her, eg; one day she likes nuddles or any other food and the other day she doesn't, even after living together for 4 years there times he can't comprehend her behavior at times (understable because its 2 people not 1), so he wants her to pursue happyness which he himself believes he cannot give her.
For some reason his girlfriend, whoever she is because we really don't know which of the twins he is going out with, never told him that she has a twin sister, so far I can only hypothesize that in the past he started hanging out with the twins without knowing they were actually two different girls or something, and they knew that both were seeing him but still kept him in the dark for some reason, until things got too far into the lie and never actually told him the truth 4 years into the relationship.
The series starts when everything is kinda falling apart, and as of ch 9 he still doesn't know of the "twin situation" but he will probably know soon enough, witnessing something he shouldn't have or the twins finally coming clean. So things might develop in the famous "they've lied to me for so long, which is a secret they don't know I know, so its not a secret to ME anymore, but I still gotta act like I don't in different situations where I'm uncomfortable, until things go down" or "welp now I've got 2 girlfriends, and now that I know my life is falling apart because I can't trust them for lying me for so long, so Netori ensues? and he gets stolen by the female co-worker and suddenly there is drama everywhere?" idk