This is book 4 of the "Skylark" series.

This note contains spoilers for the book.

Duquesne doesn't actually stay bottled up with the pure intellectuals, of course. Just as the second book ended with Seaton observing the faked destruction of Duquesne and Loring, he's back. In this case because Seaton and the Norlaminians didn't do the math, and that capsule ran so fast that it vaporized the mechanism that kept it in time stasis.

So he calls for help, and gets information from Seaton, and gives him lies intended to get him killed, and sends assassins at him through the fourth dimension. But none of it works. And at the end he actually does work with Seaton and his party, on destroying the chlorans in their galaxy and rescuing the human planets from it.

The visit to the enslaved human planet, and Seaton's "shot in the arm" for the people in the mine, and the revolution, make an excellent story. And despite what Dorothy says later, humans aren't the only species in the galaxy with a nudity taboo; it's clearly the norm on Valeron, and on Ray-See-Nee, for example.