Series in one sentence:
"The masks won't kill you, they just want you to commit suicide, but if you don't, they will kill you".
Series in more sentences:
A high school girl wakes up in a strange city, patrolled by mask-wearing killers, and comes to learn it's some kind of special dimension that tries to breed the next god.
Determined to reunite with her brother and bring an end to this ruthless system, she meets many allies and enemies along the way.
What did bother me were the abundance of panty shots and the inconsistent rules. The series starts off with the statement that masked people will never kill you, as they want you to become desperate and commit suicide, except then it follows up with "but they will kill you when you don't commit suicide". So, an entirely pointless bit of info.
I also recall hearing that the people that take off their mask or get it damaged will get targeted by other killers and feel urged to kill themselves in addition, but this rarely happened. In fact, a masked character strolls along with the main cast and is allowed to retain his memories without his mask even attempting to punish him for it, something that did happen to someone else prior.
I didn't get it, maybe I missed a bit of info, maybe the anime just isn't that logical, but it was still worth the watch.