Earth has been conquered by the Valgolian empire. There is resistance from the Earthlings, and, contrary to expectations, the empire wants to encourage this. They believe that in the process of repeatedly rebelling and failing, the Earthlings will make social progress, and eventually be prepared to take an equal place beside the empire. It is, apparently, the empire's goal to uplift all their interstellar neighbours, solving their societal problems by imposing their own order, eventually curing the conquered planets of their divisiveness and backwards attitudes.
The basic problem with Earthlings is that they don't get along with each other. People discriminate on the basis of race, nationality, religion, anything you can think of. Until Earthlings learn to treat all people as equals, they'll be under the thumb of the Valgolian empire. To ensure that they are making progress, a covert operative from the empire is modified to look like an Earthling and inserted into the resistance.
The operative gets in, falls in love with a woman, and completes his mission, looking foward to getting back together with the woman he fell for, once she has spent a few years at an indoctrination center and no longer hates him for betraying them, anyway.
In an odd choice, Anderson has the Valgolians call themselves humans, and the people of Earth call themselves human, but the two don't consider each other to be human. It is a point that the protagonist comes to recognize the Earthlings as being like himself–"They lived and laughed and loved even as humans do." And finally: "In spite of their appearance, to which I was now accustomed, they were human." I don't know if Anderson just wanted them to both use the word 'human' for 'people like me' for some linguistic reason, or if he was trying to make a broader anti-racist point with this oddity.
There are a few meaningful ideas in this one–Earthlings demand freedom, but freedom to do what? To starve and make war and die, as they did before? The Organians make that point in Errand of Mercy: when Kirk protests that they have rights, Ayelborne replies, "To wage war, Captain? To kill millions of innocent people? To destroy life on a planetary scale? Is that what you're defending?" It's hard to argue with the powerful beings imposing peace and ensuring the people's welfare, but on the other hand one does not wish to have it at the cost of self-determination. Examining exactly what it is the Earthlings want the freedom to do, and why they chafe at the rule of the Valgolians, is worthwhile.
That said, the story as a story is fairly boring. It goes on and on. The secret-spy-stuff story doesn't really do anything–Conrad just goes through the motions, occasionally interjecting a thought about how much better off the Earthlings are now than before the conquest, and eventually he ends up in position to complete his mission. But that mission seems to have changed from "incite the Earthlings to rebel" to "warn us so we can stop any rebellion" at some point without any notice being taken of it.
And the romance between Conrad and Barbara is uninteresting, to boot.