The plot: Two teen hikers, straight from Camp Crystal Lake, wander upon a secluded Top-Secret U.S. Army lab one night. Of course, they go skinny dipping in it's pool. What they didn't know is this lab houses Operation Razor Teeth, which mutates piranha to live in fresh and salt water. Their swim is eaten short. An investigator, Maggie (Menzies) happens upon this lab as she is hired to find the teens, with the help of the drunk Paul (Dillman), and they are met by the evil Dr. Hoak, played by Kevin McCarthy ("Invasion of the Body Snatchers"), but not before Maggie drains the pool into underground streams. Hoak tells them that the Army originally wanted to set these vermin loose in the rivers of North Vietnam, but since that war ended, they would wait for the next one. Realizing the piranha are now loose, Maggie and Paul race back to town where Paul's daughter is at a summer camp at the end of the river, and a new resort is opening.
If this isn't enough fun for you, the next 75 minutes are a roller-coaster of thrills. The piranha eat Dr.Hoak, a canoeist, and Keenan Wynn's legs, before reaching the summer camp and resort...then this flick is ramped-up even more. Barbara Steele arrives with the army and takes over for Hoak. She is brilliant as the mad Dr. Mengers. Complete with an English accent, she pronounces these toothy fish, pir-un-yia. Mengers was Hoak's lover, and refers to this mad-scientist as a "dreamer." Menzies is terrific as the enthusiastic yet naive investigator. Dillman, is also good as the drunk, pathetic Paul. Given the subject matter, and produced by Roger Corman, bikini clad gals, buckets of blood, and an all-out gore fest for an ending are guaranteed.
An interesting note of trivia, this story was written by John Sayles ("The Brother From Another Planet" and "Eight Men Out"). This horror flick masterfully stirs in enough humor to relieve the tension. You say "Ordinary People," and folks say "Huh? What?" Now you say "Piranha," and everyone knows what your talking about.