German U-boats are getting through the defences. It’s up to the Wrens to stop them.

After her fiancé died in the sinking of the Royal Oak, Mary finds herself stationed in Orkney, still battling her grief. With Iris and Sally by her side, she is gradually overcoming her loss and is surprised to be helped by the irascible Joe.

Joe is a signalman aboard the Kelpie and his cheeky Morse code messages to Mary while she's on shift bring her cheer and exasperation in equal measure. With Christmas round the corner, Mary is opening herself back up to the festive spirit – and to love.

But the Germans keep slipping through Scapa Flow’s defences, somehow without triggering any of the Navy’s alarms. Could someone on land be guiding them? If so, can Mary, Iris and Sally figure out who and stop them before more lives are lost?