Stanley Kubrick’s 2001: A Space Odyssey has its world theatrical premiere on April 2, 1968.

Kubrick, whose 1964 Cold War satire Dr. Strangelove had been popular with audiences and critics alike, was intrigued by science fiction but felt the genre rarely produced interesting films. He became determined to make one, using the sci-fi story The Sentinel as source material and enlisting its author, Arthur C. Clarke, as his co-writer. The film does feature a coherent plot, involving two scientists and a highly-intelligent computer sent to investigate a mysterious event near Jupiter, but several scenes—including the film’s now-legendary opening, which seems to depict hominids learning to use tools after the appearance of a mysterious monolith—are surreal and highly open to interpretation. Filming required the construction of a giant centrifuge to serve as the spaceship’s interior and numerous expensive visual effects, including a groundbreaking psychedelic sequence near the end of the film so complex that staff referred to it as the “Manhattan Project.” Kubrick is said to have removed over 15 minutes from the final cut, which nonetheless ran well over 2 hours.

Today, few would argue against the greatness of 2001, but on the night of its debut Kubrick felt he had failed. Lead actor Keir Dullea estimated that he saw 250 people walk out of the premier, while Clarke reported hearing a studio executive remark, “Well, that’s the end of Stanley Kubrick.” Some reviewers agreed, calling the film “plodding,” “immensely boring,” and even “a disaster.” Many reviews were glowing, however – Roger Ebert gave it four stars, while Charles Champlin of The Los Angeles Times called it the “ultimate statement of the science fiction film.” Audiences seemed to agree with Champlin, flocking to the film upon its release and creating such demand that many American theaters screened it regularly for over a year. The film went on to win an Oscar for Best Visual Effects and numerous other awards. Today, it is regarded not only as a seminal work of science fiction but as one of the defining films of the 20th century.