Three days of the countess who found out she is a decoy

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 saying "It would be awful if we just got married like this!" Apparently, she is supposed to be the bait to draw out a hidden villain who was threatening the duke. Grace is shocked by the revelation, but she will accept the duke's gifts and fulfill her role as decoy until the end, as she makes a plan to lure the criminal out...?