After using considerable ingenuity to evade their pursuers, Rich Evans and his school chum Sean O'Connor arrive at Boyne Castle. (See descriptions for parts 1 and 2). But the spies who have been chasing them across Ireland have gotten wise to their evasive tactics. What disguise will they use to entrap Evans and O'Connor yet again? What is Tom Evans involvement in all this? And what is the "secret" of Boyne Castle? The Secret of Boyne Castle is a wild romp through 1960's Ireland with the Irish countryside and highway system as a backdrop.