Having been run through with a Sword at Agincourt, Max has been saved by Mrs Partridge by moving her to a parallel timeline in which she is dead, but in which Leon is alive. 

Max is effectively given a second chance to create a life with Leon, forgiving and forgetting past mistakes, unfortunately, before they can do much more than realise this opportunity they are attacked by the Time Police, an organisation that in this world was created to monitor and protect the timeline, something it does ruthlessly by removing any and all anomalies. This results in Max and Leon being chased through time as they try to get away. 

Though we get to visit several interesting time periods in this novel, Pompeii and Ancient Egypt to name but two, this is different from the other books as the plot centres around the Time Police and their actions which have repercussions not just for Max and Leon, but for everyone at St. Mary's as well. 

This didn't quite reach the heights of the previous book though it does have its moments and the line "as unstoppable as a greased Elephant on a helter-skelter" will live with me for some time to come.