Hitomebore to Iwa retanoni Jitsuwa Otorida to Shitta Hakushaku Reijou no Mikkakan

Grace, a poor countess, was supposed to be happily married in three days to the high-class nobleman Duke Ansel, who professed to have felt "love at first sight". But she overheard Ansel say that it would be a disaster if they ended up married. Apparently, she was just a decoy to catch a hidden villain who was threatening the duke. Grace was shocked, but she will try to fulfill her role as a decoy until the end, and made a plan to lure the criminal out...?
(Source: Comic Zero-Sum, translated)