According to renowned physicist Stephen Hawking, "science fiction is useful both for stimulating the imagination and for diffusing fear of the future." Indeed, several studies suggest that using science fiction movies as a teaching aid can improve both motivation and achievement. However, if a movie's plot crosses the line between good and bad science, skeptical eyebrows within the mainstream scientific community begin to rise. Very little research has been conducted on whether factual errors in science fiction movies might generate or perpetuate students' alternative conceptions. This study examines students' conceptions of some key scientific ideas after watching two selected science fiction movies. Our purpose is twofold: 1) To find whether science fiction movies sometimes generate or contribute to students' alternative conceptions. 2) To examine the potential of science fiction movies as an evaluative tool for exposing common patterns of student alternative conceptions so that these may be addressed by subsequent directed instruction. It is suggested that science fiction movies are especially useful in this regard because they prompt students to reveal alternative conceptions which although common in our culture may otherwise remain hidden even if the students are directly asked. (Contains 1 table.)