Local artist and amateur sleuth Penny Brannigan has been spending her summer painting views of the once-lovely Gwrych Castle, which is now in a heartbreaking state of disrepair. So when she hears that one of the locals, Mark Baker, is leading a team of enthusiastic volunteers to restore the castle, she is thrilled. However, after disagreements over how the work should proceed break out, the body of an overbearing volunteer is discovered and Mark is implicated.
Convinced that he is innocent, Penny asks Gareth Davies, a recent retiree from the North Wales Police Service, to join her investigation. However, it is surprisingly not Gareth who finds the most important clue: while examining the estate's watercolors, Penny discovers that 90-year-old paintings might hold the key.