When a young man is found hanging from a tree strangled to death, the CSIs move in to find the killer. It appears that the man, Brian, was taking hormone injections in preparation for a sex reassignment surgery. But as usual, there's more to the story. Brian lived near a local water treatment reservoir where children often swam and fished. The recycled reservoir water was considered "grey," meaning purified, though it was not meant to be drunk.
A former hydrologist, Paul Cyden, had enlisted Brian's help to find out the truth about the reservoir. The water had been contaminated with pharmaceuticals, which had leaked into ground water in the town as well. After Paul is murdered in his home, Brian's mother is identified as Paul's shooter. She had misunderstood Paul and Brian's connection, and believed he had murdered her son. But Brian had killed himself, unable to deal with the humiliation of his misshapen body. To make matters worse, Brian's mother, ill with cancer, ultimately discovers that her cancer is also due to the water's contamination.
Sara continues to work alongside Ronnie Lake on swing shift, and they handle the case of a body found at a construction site. With Greg's help, they deduce that the man had been a reporter covering the opening of a casino (built by Catherine's father, Sam). He died with a film canister in his hands. When Greg develops the film, he discovers evidence that the gaming commissioner at the time was on the take. Sam's bodyguard, Benny Dunbar, killed the reporter to keep the news quiet. Lily, Catherine's mother, helps out on the case, and Greg takes her out to get more information from her for the book he's writing on Vegas.
Meanwhile, as Grissom continues his study of bees, he makes a proposal of marriage to Sara. She accepts.