There is poison in Cardiff Bay.

When Myrddin returned to Cardiff, the last thing he expected was for his wife to be dragged under the sea. Now, he will do anything to get her back. Can he save her without sacrificing his morals?

Nimues plan for the week was simply to solve a couple of murders and to continue her affair with that secret service agent, but the mob is growing stronger. With more corpses in her morgue, she has to chose: Will she help her brother or do her job?

Meanwhile Arthur's restaurant is failing. He has one more chance to pay back his depth to the mob before he joins his kitchen waste in the bay. Then there is his problem with the creatures at Mermaid Quay. Can Arthur save himself?

The horrors of creatures gone astray are best visible in a beautiful scenery.