When an underwater Anomaly on the Isle of Dogs opens and closes before the team can pinpoint its exact location, and a boy disappears down a flooded drain at the same time, the team are faced with an aquatic threat unlike any they've ever faced.
- Bad People Abuse Animals: Caroline, the girl who's been hired by Leek to spy on Connor and Abby, responds to Rex knocking over a salad she made by locking Rex in Abby's freezer box while Connor and Abby are out, almost killing him due to him being cold-blooded.
- Big Damn Heroes: With Abby dangling over a cliff, Connor faced with the choice of letting her go or being dragged over the edge himself and the Mer-Queen getting closer and closer, Connor throws his hand out...and Cutter grabs it, helps him pull Abby to safety, before Cutter and Stephen use the guns they brought with them to put the Mer Queen down.
- More Deadly Than the Male: While the small Mer are certainly threatening, the Mer Queen is huge and takes a considerable beating before dying.
- Oh, Crap!: The entire team when Jenny falls overboard into the River Thames, and then an all-too-familiar fin breaks the surface of the water and heads straight for her...
- Official Couple: After apparently being nixed due to time travel, this episode makes Connor X Abby canon again.
- Sea Monster: The Mer creatures
- Teeth-Clenched Teamwork: Connor and Abby go through this due to Abby's dislike of Connor dating Caroline, to the point where even the normally placid and unflappable Jenny loses patience with their bickering.
- Threatening Shark: One shows up in the episode, but it ends up dead rather quickly.
- It's possible it is a predator of the Mer.
- Wham Shot: At the end of the episode, Leek is shown having a video chat with the cleaner, while an unknown figure is sitting behind him. When he assures the figure that similar failures will not happen again, the figure leans forward, revealing itself to be Helen, who warns him that further failures will not be tolerated.
