This is book 2 of the "Skylark" series.

This note contains spoilers for the book.

Second in the Skylark series. Both Seaton and DuQuesne stumble across the Fenachrone, an old and very capable race that has had space-flight for ages but is still somehow confined to one planet, and who believe it is their destiny to dominate the Universe!

This leads Seaton to seek out older races in the Green system, eventually leading him to the Norlaminians, who know pretty much everything. They introduce him to the fifth-order rays, which can function through a zone of force, and Seaton then proceeds to exterminate the Fenachrone.

(The way the Fenachrone are portrayed, there really doesn't seem to be room for them and any civilized race in the same universe. Seaton goes to great lengths to avoid wiping out, for example, the Urvanians, even though they're threatening his friends on Osnome. But the Fenachrone need killing. One might wish to argue that no real race could be so single-minded and unpleasant.)

Along the way, we see DuQuesne and his partner reduced to ash in a Fenachrone report. But don't be surprised to see him turn up again anyway, in the next book.

The planet Dasor is also rather interesting. Mostly water, the intelligent species evolved into seal-like creatures, fully at home in the water but still air-breathing. They have a truly marvelous transportation system involving tubes full of running water that people are carried through. At the stations they just dump out into a big pool.

We also see the arms-race in super-metals -- that old Osnomian standby arenak is displaced briefly by Urvanian dagal, and then by Norlaminian inoson, which is the ultimate in possible strength. (And, in the next book, still gets moaned about. "Oh, for something really rigid to build with!"). For some reason, the Dasorians don't have a trademark super-metal.