This book represents Doc Smith at his most typical. The bad guys have become more powerful, so Seaton and Crane must bring experimental weapons and a new, improved Skylark to bear.
"Well, we're here, folks, on another new world. Not quite as thrilling as the first one was, is it?"
Seaton and Crane have developed a "Zone of Force," a shield inpenetrable by any known force or form of matter. With it in place around the Skylark, they are unable to control the ship, but its usefulness as a defensive shield is undeniable. Shortly after developing the Zone, the two encounter the evil Fenachronians, who have technology far in advance of anything the earthlings have, with the exception of the Zone of Force. The Earthlings prevail but realize that, unless they can master the Zone and learn how to project weapons beyond it, the entire galaxy will soon fall to the Fenachronians. They have little time in which to act, as the Fenachronian warship, before being destroyed by the Skylark, had sent a missile back to Fenachrone containing communications detailing the Skylark's abilities and calling for the immediate destruction of Earth. The missile will arrive in 82 days.
Another quote:
"Why attempt to do more than can be accomplished efficiently? There is tomorrow."