Mac, Stella and Dr. Hawkes investigate the murder of a man involved in a trinogamous relationship, while Lindsay struggles with a case that hits close to home when she realizes the victim, a young woman dressed as a mermaid, is from Montana. Mac and Stella are shocked when, while at a crime scene, two grief-stricken women approach them claiming to be married to the victim. The women explain they were both knowingly in a committed threesome relationship with the deceased. Meanwhile, when a young woman dressed as a mermaid is found tangled in the kelp off a ferry dock, Danny and Lindsay investigate her murder and discover that her last known whereabouts were at a child's birthday party. The case affects Lindsay the most when she realizes the victim is from her home state.