After a tumultuous school term, a family holiday with sunshine, sleep-ins and Portuguese tarts is just what Kensy and Max need. And Granny Cordelia is adamant: there are to be no investigations while on vacation. But when strange incidents start piling up, the twins are torn between their loyalty and their nose for adventure.

As all leads point to the annual E-Prix championship, Kensy and Max find themselves drawn into the middle of a menacing plot. High-powered sports cars are not the only dangers on the streets of Sintra. Someone wants Wolf Motors and the Formula E car race to go up in smoke, and they won't let anything - or anyone - get in their way.