Skylark Three opens with DuQuesne plotting evil, and Richard (Dick) Seaton, Our Hero, under siege by the minions of World Steel Corporation. DuQuesne takes off into the void to exploit a secret known only to 4 others, all dead, although for a change, he didn't kill them. Meanwhile, back at the ranch, Dunark and Sitar come on a desperate mission seeking salt and X to turn back an invasion of Kondal by the Third Planet.
On their way to the aid of Kondal, Seaton and company encounter the dreaded Fenachrone, denizens of a fog shrouded planet who's only goal is The Plan of Conquest, their plan to conquer the galaxy, and exterminate all they come in contact with. Since this is not a nice thing, after barely defeating the Fenachrone scout, Seaton assumes in fact the position of Overlord bestowed on him by Kondal in The Skylark of Space, and forces peace on Kondal and the Third Planet. He then scours the Green System for the planet that can control rays of the fifth order that can penetrate the hitherto impenetrable zone of force. And therein lies the tale.