Summer, 1941: London evacuees Harry and Rosie Morton are looking forward to a summer exploring the Lancashire coast with their new friends - and learning to tango as a surprise for Mom's birthday.

Until the twins learn two things: their awful cousin, Jasper, is coming to stay. Worse still: Dad is returning to the Western Front.

Then a train journey changes everything and suddenly Harry and Rosie find themselves caught up in a treacherous plot. And, when no one believes their story, they realize they must summon all their courage to not only protect the war effort, but save lives closer to home too . . .

With time running out, can the twins stop the crooks before it's too late and have one last dance with Mom and Dad before he leaves for France?