The Time Travellers Go to War

It's been months since Chip and Skip's first time-travelling adventure and the mysterious glasses they found in the attic of their new home - and which make it all possible - seem to have stopped working.
But Chip gives them one last try and finds herself spiralling back through time to 1940s' London and the Second World War. Evacuated with hundreds of other children to the safety of the countryside, Chip can only hope Skip will find her.
Because with a secret cave, German spies and a new friend in mortal danger, she needs all the help she can get...