The Patient Dark — Kenneth Bulmer

For something like two hundred and fifty years Earth had been dominated by humanoid aliens from the star world of Alishang. But man's spirit refused to be conquered. There was a world-wide underground planning for the day of final liberation. And there were four leaders who knew the secret that would guarantee victory - the secret of ZI.

Rupert Clinton, intelligence man for this underground, was not one of those four; yet somewhere deep in the recesses of his subconscious mind, he knew ZI's secret.