Behind the Blue Gates — Mabel Esther Allan

When eighteen-year-old Miranda Sheridan goes to Paris to join her cousin Jane she thinks she will never be happy again, for she still hasn't got over a tragic love affair. She discovers her cousin has gone. Puzzled by Jane's absence, Miranda sets out to find Jane and gradually realises that she, as well as her cousin, is in great danger.

Eighteen-year-old Miranda arrives in Paris to find that her cousin Jane has mysteriously disappeared, and that Jane's fiance Auguste is a little too anxious for her to leave.