A family outing on a paddleboat in a lake near Lyminister is devastated by the attack of a giant beast in the water. A man on shore, walking his dog watches the attack as it takes place. All ten members of the outing are killed, their boat ripped in two. The man decides to report the incident to the ARC first, knowing their line of work could solve the mystery of the beast's identity. He describes it as rusty red colour, with orange and yellow tint on the fins and a crocodilian-like head. Abbey imagines it to be a mosasaur, but Connor dismisses the thought as mosasaurs didn't have dorsal fins as the man described. Connor, Becker, Matt, and Abbey venture out on a boat into the middle of the lake, using sonar tests and cameras to search for the beast below the water. Matt chums the water, hoping to attract the monster. The idea works and the killer is soon circling the boat. Several times, its head breaks the surface, showing that Abbey was right about it being a mosasaur. Every member of the team is astonished by its gigantic size; Matt estimates it to be nearly 80ft in length. Connor decides to name it Dracostagnum rex, the king dragon of the lake. Now with the creature starting to rock the boat, probably mistaking it for prey, the team has to find a way to get to safety.