The story mainly focuses upon the exploits of the "Second Stage" Lensmen: those who have gone through the advanced Arisian training Kinnison underwent in Galactic Patrol.

This elite cadre allows Civilization to tip the balance against Boskone as Second Stage Lensmen abilities are ideally suited to spying and information gathering.

Since one of the themes of the series is that as soon as one side develops a particular weapon, the other soon figures out how to duplicate it; the Lensmen assume that Earth will be subjected to an attack using either a negasphere (negative energy that consumes anything it touches) or two high velocity planets, used like a nutcracker.

As before in this series, the ultimate weapon featured in the previous book becomes the standard for the opening stages of this one, and newer, more powerful weaponry must be developed to deal with the new danger.

In this case the weapon developed is the “Sunbeam”, where the entire output of the sun is converted to an energy beam and used to vaporize much of the Boskonian fleet when it shows up.

Kinnison's investigations take him to the planet of Lyrane, ruled by a matriarchy of women who have advanced mental abilities, but apparently little or no art, literature, music, or other cultural assets.

The Galactic Patrol is by now ready to begin a full-scale invasion of the second galaxy, so Port Admiral Haynes mobilizes the Grand Fleet.

With the patrol invading their home galaxy in overwhelming force, the Boskonians are too busy to worry about a communications line, so Kinnison and Nadreck trace it all the way back to the Thrallis system.

Nadreck takes on the Boskonian headquarters on the frigid world of Onlo and sets about carefully fomenting discord by tampering with the minds of the various Onlonians he finds there.

Since advancement within Boskonia is based on deceit and assassination, and as a second stage Lensman is much better at anything needing intelligence than the average Boskonian, he soon rises to a position just under the Tyrant.

Mentor's explanation satisfies Kinnison so he never realizes that in actuality, he has just defeated and destroyed Gharlane of Eddore, the entity who, as Nero, Gray Roger, and many others, was responsible for most of the death and misery in Earth's history.