One word: Masquerade.
Now, I know that may seem obvious, or even cliche, however. The symbolism behind a mask is so much more than just a mask. My plot idea would be exactly what I stated above, a masquerade back when it was popular in its time. A romance, of sorts. But one that's filled with murder and spite. Two lovers, absolutely stricken for each other, however, in a masquerade the male partner finds himself flirting with a woman who is not his partner. The lady in the mask whom was involved in the flirtatious conduct, gets murdered in the evening. No one knows who caused it, but the deaths hence forth are all murders of women from masquerade balls.