An Arab rolled a deserter's head in the sand, waiting for his reward, in the desert fortress of El Zeeb... Every legionnaire became suspicious of his neighbour and hatred led to other severed heads rolling. First to follow was that of Captain Paon, the Commander. The murder seemed impossible: his room was locked from within; there was a thirty-foot drop from the window; and the tower, detached from the fortress, was under constant surveillance by a guard.