In 1970 three kids are born during a total eclipse. Because of this, with the sun and moon blocking the planet Saturn, which controls emotions, they become psychopathic killers a decade later. Two of them are boys, one is a girl. The girl's father is the town's sheriff, and she and her two friends dispatch him with a baseball bat, making it look as if he fell on the porch steps. She even kills her older sister (Julie Brown of EARTH GIRLS ARE EASY) with a bow and arrow through the eye.
A ten year old boy and his older sister, Joyce (Lori Lethin of GOLIATH AWAITS) suspect that something is wrong with the maniacal trio and have to try surviving to the end of the movie. There's a brief appearance by Jose Ferrer as the Doctor who had delivered them all. BLOODY BIRTHDAY is a decent enough "Killer Kid" flick but it's more notable for being an early "Slasher Movie", complete with Friday the 13th-esque music cues.