Triton is Neptune's largest moon located in Sol System, and one of the largest in the Solar System. Triton orbits Neptune in a retrograde orbit which is in the opposite direction than Neptune's rotation. Triton is thought to be originated from the Kuiper Belt, captured by Neptune. Humans sent a probe on a flyby of Triton in 1989.
In the science-fiction story, humans sent scientists to Triton in the 2080s to investigate it