When an anomaly triggers in the tiny coastal town of Witchfield Cove, Matt (Ciaran McMenamin), Abby (Hannah Spearritt) and Connor (Andrew Lee Potts) are sent to check it out. The anomaly reaction is spotty which makes Connor think it is unstable. When they arrive, they visit the local pub to see if anyone will talk to them. A framed article on the wall of the pub makes a reference to a local legend called the Witchfield Worm. They also hear about a camper who has gone missing. The team suspects that the mythical Witchfield Worm is actually a creature that came through an anomaly.
Matt finds the remains of the camper and runs into the Witchfield Worm, which turns out to be a Labyrinthodont. Matt gets called away when Jess (Ruth Kearney) finds out that Ethan (Jonathan Byrne) has kidnapped Emily (Ruth Bradley). Jess presses James (Ben Miller) into service when she finds herself stretched too far trying to keep track of Abby trying to find the creature, Connor trying to find the anomaly and Matt trying to find Emily. With Jess helping Connor and Abby, James is left to coordinate with Matt.
In the pub Moira Lennon (Anne Kent) and her son Ray (Brian Gleeson) hear that Abby, Connor and Matt are looking for the worm. They are aware of the anomaly and have been using it for their own reasons.
Meanwhile Matt tracks Emily and Ethan to the cemetery. Ethan, looking for revenge and blaming Emily for Charlotte’s death, buries her alive in the Cameron crypt. Gideon (Anton Lesser) believes that Ethan may pose a danger to humanity, like Helen Cutter did.
Philip Burton (Alexander Siddig) offers Connor a job working exclusively for his company Prospero Industries.
Episode 5 of season 4 features a Labyrinthodont. Labyrinthodonts were large amphibious creatures which lived in the Palaeozoic and Mesozoic eras. It was a semi-aquatic carnivorous amphibian with a long serpentine body.