Rachel and Michael end up in a cold, loveless marriage after she tries to elope with another man on the eve of their wedding. She feels guilt and shame and thinks he's only married her to uphold the family name - and he doesn't want to force himself on her although he loves her still.
The frozen state of their marriage starts to thaw when Michael's involvement in a Bow Street investigation leads him to the supposed murder of Rachel's former love's wife. Rachel gets drawn into the investigation and Michael employs his mastery of disguise to win the wife he secretly loves.
The thawing of the relationship is well done (though you can't believe they've gone so many years being married to each other without seeing what's under thier noses!) The attempted seduction of Rachel by Michael under the guise of his half-brother is fun, and Rachel's development from seemingly cold-hearted prig to a woman of action is also good. Michael is the better character of the two - I did find myself getting quite irritated with Rachel in the earlier part of the book. However, once it gets going you do find yourself caring what happens to them - always a sign of a good book!