In the town of Mansi on the shores of Lake Champlain, it's Columbus Day weekend. There's a water travel ban due to the fog, and Rose Phillips prepares to go out with a camera. Her mother Leigh asks her son Joseph how the dishes that he's washing are coming, and Rose shows Leigh her collection of clippings about "Champy" the lake's fabled monster. Leigh says that they had an understanding that Rose would never talk about Champy, and insists that the monster doesn't exist. The foghorn sounds, and Joseph mentions their father William's theory about it and Champy. Leigh tells Joseph to finish up and warns about how her new husband Chet gets when he's in one of his moods.
Rose looks at photos of William with the family, and then quickly hides as her friend Thomas comes to the door. She tells Leigh that she can't let Thomas see her like she is, but Leigh answers the door. Thomas asks if Rose is home, and Leigh tell him that it isn't a great time. As she starts to close the door, Chet wakes up and comes out. Chet notices Thomas' knife and tells him that it's one thing to have it and another thing to know how to use it. He takes the knife and talks about when he was Vietnam, he used a similar knife to kill. Chet puts the blade to Thomas' throat and then traces it down his chest, and talks about he would twist it to make a man suffer. He gives the knife back to Thomas and says that there's only one rooster in the henhouse, and tells Thomas to get out and stay away from Rose.
Chet goes inside and tells Leigh to get him beer. Leigh serves beer and meat loaf, and Chet complains that the meat loaf is cold. He says that he was out all day trying to find work and support the family, and Rose tells him that he was at a bar drinking. Chet grabs her notebook of clippings and mockingly says that it's the nonsense that killed William. He throws it in the garbage can and says that's where it belongs. Leigh tells Chet that he didn't have to throw the notebook away, and Chet says that he was trying to help and goes out to work on his truck.
Once Chet is gone, Rose gets her notebook. She asks why Leigh puts up with Chet, and says that Chet took advantage of Leigh when her husband's insurance paid off. Leigh tells Joseph to go out and play, and then tells Rose that her husband wasted his life chasing Champy and everyone thought he was crazy. He spent all of his time and their money and forgot about them, and it got him nowhere. William ended up dead in the lake, and Rose says that her father didn't care if the town believed him. He wanted Leigh to believe in him, and Rose runs out.
Rose goes to the lake and Joseph, seeing her, follows. Thomas is watching and goes after her as well. She gets to the lakeshore and bumps into a giant lizardlike carcass. Thomas and Joseph arrive, and Joseph finds its head at one end. The foghorn sounds, and Rose figures that William was right and the creature is Champy. It's smaller than William father thought, but Rose insists that it's Champy. The girl figures that Champy is some kind of dinosaur, and Thomas says that she'll be famous. Rose figures that William found her first and she wants him to get the credit for it. She tells Joseph to get Leigh so she can see the proof that William was right.
Once Joseph leaves, Rose takes pictures of Thomas standing next to Champy. Meanwhile, Joseph runs home and Chet sees him. The boy yells to Leigh that they found Champy, and Chet goes to see.
Rose talks about how one day William disappeared and Leigh called the cops. The next morning when the mist broke, Rose found him washed up on the beach unconscious. When William came to, he looked at his family like strangers and didn't say anything. One night at dinner, he said that Champy was real. Leigh begged William husband not to say anything for fear people would think he was crazy, and he lost everything while searching for the creature. One morning he was washed up on the beach again, dead. Thomas tells her to stay there and walks over to Champy with his knife and carves "William + Rose" into its skin. He tells her to take a picture of the words so that they can get William's name into National Geographic.
Thomas takes Rose's hand, and William walks between them and stares at Champy in astonishment. He says that he'll finish the job that William started, and figures that the corpse is his ticket to riches. Chet grabs the photo from Rose, crushes it up, and grabs Thomas when the boy tries to stop him. He takes Thomas' knife and shoves him away, and puts it to Thomas' throat when the teenager tells him to stop. Chet throws the knife into the sand at Thomas' feet and dares the teenager to pick it up and come at him.
As Thomas reaches for the knife, something roars in the mist. While Thomas is distracted, Chet knocks him down and then kicks Thomas when the teenager comes at him. When the man picks up his knife and prepares to kill Thomas, Rose grabs Chet's arm and bites it.
Joseph grabs Leigh's hand and tells her that William was right. Leigh wonders where Thomas and Rose are, and they hear Thomas and Chet fighting.
Chet throws Rose down next to Thomas, grabs his knife, and says that they're in it for it. Rose refuses to go without Thomas, and the dead creature's much larger mother emerges from the lake. It eats Chet whole, and Rose watches as the real Champy nudges its child. Champy looks at the teenagers for a moment and then pulls its child into the lake. Rose figures that no one will believe them with the evidence gone.
Leigh and Joseph arrive, and Rose tells her mother that Champy was there and William was right. Leigh doesn't believe them, but then catches a glimpse of Champy out on the lake. Smiling, she realizes that William wasn't as crazy as everyone thought he was, and Rose hugs her. They go home together, and Chet's bloody severed foot washes up on the shore.
Written by Gadfly on Dec 5, 2019