Two women both claim to be the wife of a murder victim surprise Mac and Stella at the circumstances of their relationships. Both women have been in a trinogamous marriage with the victim. Meanwhile, Danny and Lindsay investigate the murder of a young woman who is found off a dock tangled up in kelp while she is dressed as a mermaid.

A young woman dies after she starts bleeding from the eyes, nose, and mouth while at a party aboard a subway train. Mac originally believes she is the victim of a biological attack. Later, it is revealed that Sheldon knew the victim. Across town, a wealthy playboy is found dead in an indoor lap pool in his penthouse suite. The investigation reveals that he was murdered and his last maid was fired two days earlier.

At a French Revolution themed fundraiser at the UN, specifically at the French consulate, Mac, Stella and Hawkes find themselves hurdling diplomatic road blocks investigating two murders. Meanwhile, Danny and Lindsay reunite on a “blinged” out cockroach that lead to murder.

Trouble begins for Mac when Clay Dobson becomes a free man due to a mistake he made. Stella learns the results of her HIV test from Adam.

Sheldon Hawkes goes under water to gather clues but could be in danger when he can't come up for air. Mac learns of something else under water.

The team has two cases tonight and two great tourist locations: Madison Square Park where a man is stabbed to death, and the Metropolitan Museum of Art, where a woman is shot dead on the steps.

What is even more perplexing about these cases is that the team soon realizes that these two victims hold something criminal in common.

A woman is found in the trunk of a crushed car at an auto wrecking yard, the autopsy reveals that she was pregnant, the team needs to find out who she is so they can find the baby.

When the Deputy Mayor's dead body falls from the ceiling, along with hundreds of balloons, at a charity fundraiser, Stella and her date find themselves in the middle of the crime scene and Mac goes head to head with Robert Dunbrook.

While the team investigates the explosion of a gourmet food truck, Sheldon's new relationship could get him in big trouble at the lab.

When the CSIs investigate two seemingly unrelated deaths, their separate trails of evidence lead to a surprisingly connected conclusion.

When a psychiatrist's patients are being systematically murdered, the CSIs are forced to play along in a game of Clue in order to catch the killer.