Verity Grey is thrilled to be asked to join archaeologist Peter Quinnell's dig in the Scottish borders, but after her first day she isn't so sure. Quinnell, having spent his whole life searching for the resting place of the lost Ninth Roman Legion, is convinced he's finally found it - not because of any scientific evidence, but because a local boy has seen a Roman soldier walking in the fields, a ghostly sentinel who guards the bodies of his long-dead comrades.
The worst thing is that Verity believes in Peter, and the boy, and even in the Sentinel, who seems determined to become her own protector...but from what?