A teenager, Lucien (Jacob Anderson), on the Isle of Dogs, is pulled down into a sewer drain. At ARC the anomaly detector sounds. Before they can pinpoint the location, the anomaly closes. Going by the basic coordinates the team locates the sewer drain. The sewer leads to a canal. During a search of the canal Jenny (Lucy Brown) is knocked into the water and attacked by a shark. Stephen (James Murray) shoots the shark, saving Jenny’s life. The shark is not from the present or the past but a future evolution. An autopsy shows no evidence of the teenager. This means there is something else in the water.
During the search, sounds were heard that couldn’t come from the shark but from a mammal with vocal cords. Nick (Douglas Henshall) believes the animal is no longer in the canal but in the river Thames. Stephen disagrees. James (Ben Miller) takes Nick off the case and puts Stephen in charge. Still believing the creature has moved, Nick, Connor (Andrew Lee Potts) and Abby (Hannah Spearritt) concentrate on the river. They record the unusual sounds and then play them back to see if it gets any results.
The unknown aquatic creature turns out to be a Mer Creature from the future. It takes Abby off the boat and drags her down into the water. A search turns up nothing. On their own, Nick and Connor continue the search for Abby. As they search a warehouse near the river, the anomaly opens again. Eventually they find not only Abby but Lucien as well. Lucien is rescued but the Queen Mer Creature takes Abby through the anomaly. Conner goes in after her.
Oliver (Karl Theobald) connects with a Special Ops guy that is only referred to as The Cleaner (Tim Faraday). The Cleaner has been spotted three times by people on Nick’s team and Oliver is worried that the man will compromise his plans. After their conversation, Oliver is questioned by Helen (Juliet Aubrey) who wants her own assurances that their devious plot will not be exposed.
Episode 4 of season 2 features Mer Creatures and a Future Shark. The Mer Creatures are aquatic primates from the future. They resemble sea lions or walruses. They have muscular torso and arms with webbed hands. The torso ends in a large tail. The Mer Queen is much larger and has a reddish tint to its markings. There is speculation that Mer Creatures were once humans that have evolved back to aquatic creatures.
The Future Shark is similar in general appearance and size as modern sharks but with spines on its back. The main different is the development of an extendable proboscis that looks like a giant tongue with teeth. With it the shark can reach out and grab its prey, pulling it into its mouth. It is one of the few creatures that is not CGI.
There is a Wilhelm scream when Connor is hit by the Mer-creature.