Olaf and Hannah Stein are the cleverest people in the world. They have developed the MI9 computer firewall and no one else in the world can crack it, so when they are kidnapped, the spies are left holding their baby, Grace.

An archaeological dig has come to St Hope's to investigate the mysterious disappearance of a Roman legion, and it is getting worryingly close to HQ. The spies must stop the archaeologists from uncovering HQ.