Inventors and the City of Stolen Souls by Alexander Gordon Smith
Nate and Cat thought they had destroyed the evil inventor Ebenezer Saint. But they were wrong. Now everybody seems to be acting strangely. It's only when Nate and Cat witness their friend Allan being struck by a car that their worst fears come true. The impact should have been fatal, but the boy simply gets to his feet and walks on - oblivious to the gleaming metal visible through his torn skin. The young inventors know one of Saint's robots when they see it, but how will they defeat them this time?