context: David (Roger Richman) and Barbara (Janie Squire) are two young backpackers who happen upon a military post that seems to be abandoned. They discover a large man-made reservoir and decide to go skinny dipping. Within minutes they are attacked and consumed by some underwater presence, their death screams attracting the attention of somebody still inside the military facility.Maggie (Heather Menzies) is an investigator hired by Barbara's parents to find their daughter and her companion. With financing from her employer Earl (Richard Deacon), Maggie travels to the mountainous area where the two were last seen, and she seeks out the cabin of Paul Grogan (Bradford Dillman). Paul is a backwoods type who lives in a log cabin with his young daughter, who is away at summer camp. Maggie convinces him to take her to the military facility, and they discover the pond. Paul notices that the water is salty. Exploring the buildings of the complex, they find a laboratory with numerous strange animals, including a bizarre, lizard-like creation. Maggie decides that it's possible that David and Barbara drowned in the pond, and she takes the initiative to drain the pond to find out. She flips the switch, and suddenly they are confronted by a white-haired man (Kevin McCarthy) who lurches at them in a panic, trying to shut off the drain switch. In the melee, he is knocked unconscious.Paul and Maggie go outside to the empty pond and find the bones of David and Barbara caught in the drain. When the mysterious man attempts to steal Maggie's jeep, he crashes it and injurs himself even more. Taking him down to Paul's cabin for the night, the man rables on about 'razorteeth' and that they are all in danger.The next morning, Paul and Maggie load the man onto Paul's homemade log raft and they set out downstream to take him to town. When he awakens, he reveals that he is Dr. Hoak, and he explains that Paul and Maggie have done something terrible. The pond contained a breed of mutant piranha, which Hoak had been developing for use in the Vietnam war. After the war, the military pulled the plug on the operation, but Hoak remained and continued experimenting with the piranha at the facility after it was closed down.The piranha begin to claim victims immediately. Paul's friend Jack (Keenan Wynn) dies when the piranha chew off his feet while he sits on the edge of a dock; he crawls away and bleeds to death. Downstream, a father and son are out fishing on the river; the father is killed while trying to fix the outboard engine from under the boat when he is pulled under the water and eaten alive. The boy clings to the capsized boat, but it is gradually sinking into the water. Paul, Maggie and Hoak come upon the scene and the doctor leaps into the water to save the boy. He manages to lift the child onto the raft, but the piranha attack him. Paul and Maggie pull him out of the water, but his wounds are too severe and he dies before he can offer them any advice about how to stop the killer fish.Paul is especially concerned about reaching the summer camp where his daughter, Suzie (Shannon Collins), is located. The camp is downriver from the piranha, and it is only a matter of time before the fish reach it. There is also a new resort further down the river that is about to open for business. Grogan and Maggie make frantic phone calls to both places, but their warnings are dismissed as fiction.Eventually the military gets wind of the piranha and they send Dr. Mengers (Barbara Steele) with a group of soldiers to investigate. At first, Mengers denies the piranha exist, but Maggie soon realizes that Mengers and Dr. Hoak were once lovers, and Mengers represents the military's desire to cover up the incident. The soldiers confine Maggie and Paul to their camp, imprisoning them and refusing to let Paul contact his daughter.Maggie and Paul escape the camp, but they are detained by the local police before they can get anybody to listen to them. Paul does manage to contact the head counselor at the camp (Paul Bartel), but he dismisses Paul's message due to his reputation of being an alcoholic. Maggie and Paul manage to break out of jail by subduing the officer who is left to watch them, and escape in his squad car.The next day, the piranha attack the summer camp. Suzie, who is afraid of the water, is hiding on the shore when the other kids and counselors begin to panic in the water. Suzie bravely tries to rescue her two friends, counselors Betsy (Belinda Balaski) and Laura (Melody Thomas Scott) by rowing out onto the lake in a rubber raft. She collects Laura, but Betsy falls into the water and is consumed by the piranha.Paul and Maggie arrive at the camp too late, but after ensuring that Suzie is alright, they race off to the resort. The owner Buck Gardner (Dick Miller) refuses to answer their phone calls (under the advice of Dr. Mengers and her commanding officer who is there), but eventually the piranha attack and mass casualties result as dozens of people are wounded or eaten alive by the vicious fish. Mengers superior is one of the casualties when he falls in the water as people cling onto a ferry boat and he is eaten alive along with several others.During the drive in the stolen police car, Paul remembers that the water in the test pond was salty, but the water in the rivers is fresh water. They conclude that Hoak had bred the piranha to survive in either salt or fresh water, hence if the piranha reach the ocean, they will contaminate the world's waterways, and probaly the oceans as well.Paul and Maggie arrive at the resort and comandeer a speedboat to take them down the lake. Paul's plan is to poison the fish with pollution from a nearby sewage plant, but when they reach the plant by boat, it is now underwater, having been flooded. Paul intends to swim for it, tying a rope around his waist and instructing Maggie to take off if he has not surfaced within two minutes. He swims down to the plant and manages to get inside, opening up the tanks containing the raw sewage, but the piranha approach and attack him. Maggie finishes her countdown and takes off, pulling Paul to safety.Paul is wounded and in shock, but alive. He is tended by Maggie and Suzy at the resort as the military and other medics tend to dozens of dead and wounded people. As the film closes, Dr. Mengers speaks to the media, assuring everyone that there is nothing left to fear of the piranha whom have been killed by the sewage waste at the edge of the lake. But, the open-ending question leaves the end up in the air: have the piranha really reached the ocean as was once feared?
question: What do they do in the swimming pool?
answer: Skinny dip