Lieutenant Ivor Loveday returns from survivors' leave after being torpedoed in the Atlantic. It is 1941, and Nazi U-boats are sinking Brtish shipping at an alarming rate.

Ivor takes up his new appointment as First Lieutenant of HMS Hosta, a decoy, or Q-ship, brought out of retirement and hastily refitted after similar service in the previous war.

He is a capable and confident officer, and he sets about training Hosta's gun crews and performing his duties as First Lieutenant.

At Western Approaches HQ in Liverpool, he meets Grace, a confident and outspoken Wren from Tyneside. Their mutual attraction is immediately apparent, and their relationship flourishes until Grace begins to have reservations about their social differences and their future. He disagrees, but she is a determined young woman.

Against the backdrop of the Battle of the Atlantic, Ivor can have no idea whether he has a future with Grace or if they are destined to part. Meanwhile, he must concentrate on the task allotted to him, to sink U-boats.