Ransom on the Riviera

Book 4 in the Christie and Agatha's Detective Agency series!
'If you want to see your cousin again, come to the old quay at seven tomorrow night, alone. Be prepared to be married. Do not involve the police.'
A relaxing break on the French Riviera becomes a dramatic investigation when famous physicist, Irène Curie, receives a ransom note demanding her hand in marriage. What’s more, Christie and Agatha’s friend has been kidnapped! Can the twins find out who is behind the ransom note and save both Auguste and Miss Curie?