Following a humiliating experience involving the man she thought she'd marry, Iris Tredwick signs up to the Wrens in order to escape - and please her mother by finding the right sort of man. After a bumpy start, she befriends outspoken Mary and dreamer Sally as they are sent to their first posting in Orkney. There she meets mechanic Rob, whose flirtatious nature both charms and confounds strait-laced Iris. Much more appropriate for her is local doctor Stewart - if only she felt the same spark for him as she does for Rob... As Iris, Mary and Sally work to interpret signals from incoming ships, they realise the enemy is somehow one step ahead of their manoeuvres, dropping sea mines under the cover of darkness. Could there be a spy on the island? And can the Wrens prevent disaster striking before it's too late?