The death of construction worker, Pete Riggs - during an underground explosion takes the CSI's seven hundred feet below ground into the world of "Sandhogs", a union of construction workers who build all underground tunnels in Manhattan. Autopsy reveals Pete died from Asthmatic Asphyxiation before the explosion and Mac and Danny find an empty inhaler and evidence Pete was intentionally left underground, suggesting a cover-up. They question the Sandhogs, including the Foreman and Pete's brother, Joe Riggs, who both claim it was an accident. A drop of Pete's blood is found on a Sandhog's boot and it is revealed fellow worker, Tom Zito, got in a fight with Pete for carelessly dropping his carabineer down the tunnel opening, striking Tom, leaving permanent mental damage. Later, Joe is hospitalized after a beating, but refuses to divulge any details. Mac re-examines Pete's injuries and notices his finger is dislocated and has a void from a missing Sandhog union ring. Mac suspects the Sandhogs beat Joe for breaking their "code" by leaving his brother underground, and ultimately Joe admits he was trying to teach his brother a lesson about respecting the site, only to discover his brother dead the next morning. Joe panicked and attempted to cover-up Pete's death with the explosion. Also, Stella and Aiden investigate a sixteen-year-old "floater", Hannah Recchi, found with ligature marks on her neck. They determine she was clubbing with her friend Tina Paulson at Tina's older brother Matt's bar and video surveillance shows Hannah was intimate with him. A visit to the Paulson house reveals Tina's deceased father was a hunting buff, and Stella finds a picture of Hannah with Tina's dad and brother, being "blooded" for killing her first fox. Tina's purse strap matches the pattern on Hannah's neck and has her blood on it. Tina couldn't stand Hannah winning her father's attention, and now her brother's, so she ultimately confesses she strangled her.