Sandy Watson is an eight-year old blind piano prodigy who like her mother was kidnapped and held hostage so that Captain Mike Watson, Jill's husband and Sandy's father could use his connections to steal money from the government in exchange for the lives of his wife and young daughter who their kidnapper threatened to kill if the deal didn't go through.
Despite being under orders not to leave, Mike Watson managed to get a message to his secretary, Shirley Wilkes who in turn went to NCIS, alerting Gibbs and his team to what was going on while they personally took control of the case.
Sandy was later released as a sign of good faith, and was picked up by NCIS.
As a result of her blindness, Sandy's other senses, particularly her hearing, are more sensitive than normal, which becomes vital in locating where her mother is being held. She hears background noises in the video of her mother, including a passing-by train, which allows the team to pinpoint the location as they know the exact time of day it passed by (from the video's timestamp).
The kidnapper, a former Petty Officer named Kyle Grayson was later taken into custody and it was also revealed that Captain Watson himself had arranged the abduction all so that he and his family could get away with the ill-gotten two million dollars.
Presumably, Captain Watson was arrested and imprisoned for his crimes while it's possible that Sandy and her mother, Jill moved somewhere else to make a fresh start for themselves after the ordeal.
Despite being blind, Sandy has shown that she has skills far beyond what the average human being possesses with this skills later helping the NCIS team arrest the kidnapper and save Mrs. Watson as well.
Despite being blind, Sandy is observant with her other senses being of great help. She can identify a Hershey chocolate bar by its scent before Ducky even pulls it out of his pocket, and can also hear a whisper across a busy room.
As a piano prodigy, she can also identify the frequency range of any sound, which comes in handy when she hears important background noises in the audio of a video recording, and allows her to instruct Abby in adjusting the audio so that the rest of the team can hear them as well.