Ciaphas Cain is back, with his usual account of how his reputation is all sham, and he's a self-serving dirty coward, and the usual sort of evidence that he is not telling us the full truth -- in this book, Amberly's footnotes tend to comment that he never seems to notice that he's the great hero Ciaphas Cain, and someone might listen to him because, well, it's him.
But it opens with his attempt to escape the tyranid invasion having landed on him a planet about to be overrun with the tau (whose catch phrase is "for the greater good" -- whence the title.), and indeed in the middle of a last stand against them, but abruptly, in the middle of the battle, a tau contacts him to carry a message to the commander: that they want an immediate truce.
Furthermore, it turns out that they mean it -- they do not attack Imperial forces once it's settled -- which brings the Imperials to the negotiating table very quickly. They want a truce while both sides defend their planets against the assault of the tyranid fleet. Given that the tyranids are insectile monstrosities with the aim of consuming all available biomass on the planets, the Imperials agree. Cain ducks out of the task of being the tau's human liaison, because the fleet seems to be going that, and heads himself off with the imperial forces to a forge world that is one of the fleet's less probable attacks.
This being Cain, you can guess what happens. The rest of the story involves a startling discovery about research being done there, a crash landing in ruins, an ominous message from an astropath, and a crucial leap of logic by Cain.