A third episode of the series Primeval
Cutter is desperate to make Lester realise the seriousness of the situation, but he remains infuriatingly optimistic, hoping that this problem will disappear as quickly as it arrived. But he’s not in luck.
The arrival of a vicious sea creature, a Mosasaur, at a local pool is bad news for a pair of young lifeguards. The boy becomes the Mosasaur’s next meal and the girl is charged with his murder. However, Cutter realises the truth when the boy’s regurgitated body is found in a nearby reservoir; they’ve got a prehistoric killer in their waters.
The anomaly has moved from the swimming pool to the depths of the reservoir. A diver is lost into the darkness of the anomaly, and another Mosasaur is unleashed and attacks Abby. Connor fends it off.
Cutter thinks the anomaly is running along a kind of temporal fault line, but Lester is quick to dismiss his ideas. Meanwhile a young mother (who lives along the predicted line) is in the basement when she notices a small leak, which quickly turns into a flood. She sends a plumber down to investigate, but a bird-like creature from the Cretaceous know as the Hesperonis is waiting, ready to pounce. It attacks and eventually kills the plumber.
Lester personally arrives on the scene in an unmarked car and orders Nick to go through the anomaly with an armed team and bring Helen back. Nick goes through but alone.
He comes ashore and puts his scuba down on the beach. Pterosaurs  are flying about. He climbs over some rocks which are a Hesperornis rookery, and finds Helen swimming nude in a shallow enclosed pool, and calls to her. Helen answers him as if expecting him, and comes out and dresses. Two Hesperornis investigate, and she shoos them off by throwing small stones, and says that they are aggressive but stupid. Nick and Helen then have a long discussion where he tries to convince her to come back to the modern world with him. She counters, saying that she has been exploring the anomalies, seeing many wonders, and wants Nick to join her in that time-wandering life, but Nick cannot forgive her for letting him think she was dead all this time.
Enraged by his refusal, she claims that he should have moved on after her apparent death, like she did, and that the people who die on the streets of London are just nobodies. Nick realises that the Helen he knew is gone and goes back alone, much to Helen’s disgust. While swimming for the anomaly, the mosasaur attacks. He fires a harpoon at its head, but its skin is too thick to penetrate. He keeps dodging it as it charges but it rips open his scuba leaving him without air. As the mosasaur swings round for its final attack, a bigger mosasaur arrives and attacks it. Nick, who is towed back through the anomaly by the rope, needed artificial respiration but lives. When recovering he sees that Lester has sent Captain Ryan and an armed team through the anomaly and have brought Helen Cutter back by force.
1.Mosasaurs were smooth and streamlined; the one depicted is covered in Crocodilian armor.2.Pterosaurs are sometimes referred to in the popular movies as dinosaurs, , but this is incorrect. The term “dinosaur” is properly restricted to a certain group of terrestrial reptiles with a unique upright stance (superorder Dinosauria), and therefore excludes the pterosaurs, as well as the various groups of extinct aquatic reptiles, such as ichthyosaurs, plesiosaurs, and mosasaurs (the one in this episode). Read more about the some mistakes in the popular media with the real dinosaurs. 3.There is no evidence Mosasaurs were cannibals, nor that they swallowed prey whole. Source: Primeval wiki