The story picks up immediately where Gray Lensman left off as Kimball and Clarissa are heading off to get ready for their impending marriage. Mentor of Arisia stops them by commanding Kim to "think" before he acts, and Kim, of course, immediately realizes that the Boskonian organization was probably not destroyed when Jarnevon was cracked between two other planets and still poses a grave threat to Civilization. The wedding is put on hold as Kinnison and the other Lensmen set about coming up with a defense for the expected attack upon Earth. 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.