The Enterprise takes a group of ambassadors from the planet Orakisa to look for a long lost plant that will save the lives on a colony world. They are searching among the Orakisa's sister worlds, all founded by the same planet.
Their last hope brings them to a solar system with two sister worlds, but the differences between the two worlds are astonishing. One is a highly advanced world that lacks only interstellar flight to join the Federation. The other world is primitive at best with the population struggling to survive and dying of diseases that could easily be treated on the other world.
When the Enterprise crew begins investigating the primitive world, things get dangerous.
This is a good Star Trek novel. It manages to be different and believable. And there is a very funny subplot involving Worf and a hamster.