Following a humiliating experience involving the man she thought she’d marry, Iris Tredwick signs up to the Wrens in order to escape and - hopefully - find ‘the right sort’ of man to please her mother. After a bumpy start, Iris manages to befriend outspoken Mary and dreamer Sally as they are sent to their first posting - in Orkney.

There she meets engineer Rob, whose flirtatious nature both charms and confounds straight-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 always one step ahead of their manoeuvres, dropping sea mines under the cover of darkness.