[5] The coastal town of Bristol Cove, Washington, known for its legends of once being home to mermaids and mermen for centuries, is turned upside down when a mysterious young woman named Ryn Fisher (Eline Powell) appears and begins wreaking havoc upon the small fishing town to look for her captured older sister Donna (Sibongile Mlambo) who was abducted at the hands of the local military.

Marine biologists Ben Pownall (Alex Roe) and Maddie Bishop (Fola Evans-Akingbola) work together to find out who and what drove this primal hunter of the deep sea to land.

By Season Two, more merpeople started appearing in Bristol Cove due to the pollution in the nearby waters and the sonic waves from the oil rig.

In Season Three, Ben, Maddie and Ryn contend with the mermaid Tia (Tiffany Lonsdale) who plans to coerce all merpeople tribes/colonies to join her in her fight to overthrow humanity.

To further escalate matters, Ted Pownall has finally acknowledged the existence of merpeople and seems to be on a dangerous path, just as his paternal great-great-great-great-grandfather Charles had.

Besides sisters Ryn and Donna, the following mermaid and mermen characters are: When humans and merpeople secretly mated over the centuries, they produce children with a varied blend of bloodlines, from one-eighth, to one-quarter, to half of both species.

Cami, Vivian, Levi, Eliza and Ryn's mate ask to use Calvin's motorboat for a sneak attack on the enemy.

Ryn is ready to face the rogue mermaid while Ben goes to save Hope with his newfound water-breathing abilities from the stems cells he has been injecting himself with.

[5] On August 24, 2016, Eline Powell joined the cast in the role of Po (renamed Ryn) and Rena Owen as Helen.

The website's critical consensus reads, "Siren turns traditional lore on its tail with a unique, well-paced show that presents dangerous, violent mythical creatures in a surprisingly empathetic and exciting light.