The Silvers

What humans want from the Silver Planet is water. What they find is a race of humanoids who are sentient, but as emotionless and serene as the plants and placid lakes they tend.
B, captain of the mission, doesn't believe that the "Silvers" are intelligent, and lets his crew experiment on them. But then he bonds with Imms, who seems different from the others-interested in learning, intrigued by human feelings. And B realizes that capturing, studying, and killing this planet's natives has done incalculable damage.
When a fire aboard B's ship kills most of the crew and endangers Imms, B decides to take him back to Earth. But the simplicity of the Silver Planet doesn't follow them. Imms learns the full spectrum of human emotions, including a love B is frightened to return, and a mistrust of the bureaucracy that wants to treat Imms like a test subject, even if they have to eliminate B to do it.