Doppelgänger (released internationally as Journey to the Far Side of the Sun) is a 1969 British science fiction film written by Gerry and Sylvia Anderson and Donald James, produced by the Andersons, and directed by Robert Parrish. Filmed by the Andersons' production company Century 21, it stars Roy Thinnes, Ian Hendry, Lynn Loring, Loni von Friedl and Patrick Wymark. Set in the year 2069, the film concerns a joint European-NASA mission to investigate a newly discovered planet which lies directly opposite Earth on the far side of the Sun. The mission ends in disaster and the death of one of the astronauts, following which his colleague realises that the planet is a mirror image of Earth in every detail, with a parallel and duplicate timeline.

The film was the first major live-action production by the Andersons, known for their puppet television programmes such as Thunderbirds. Having originally conceived the story as a television play, they were encouraged by their employer Lew Grade to pitch the project as a feature film to Jay Kanter of Universal Pictures. Though underwhelmed by the script, Kanter greenlit the film after the Andersons hired Parrish as director. Doppelgänger was shot between July and October 1968 at Pinewood Studios and on location in England and Portugal. The relationship between Parrish and the Andersons became strained as filming progressed, while creative disagreements between Gerry Anderson and his business partner John Read, the film's director of photography, led to Read's dismissal from Century 21. In an effort to distinguish Doppelgänger from their puppet productions, the Andersons wrote adult themes into the script, although cuts were required for the film to be awarded an A certificate by the British Board of Film Censors.

Doppelgänger premiered in August 1969 in the United States and October 1969 in the United Kingdom. It performed poorly at the box office during its initial theatrical run but has since garnered a cult following. The film has received mixed reviews from critics; while the special effects and production design have been praised, some commentators have judged the parallel Earth premise to be clichéd and uninspired. Various plot devices and imagery have been viewed as pastiches of other science fiction films, such as 2001: A Space Odyssey (1968). Several members of the cast went on to appear in UFO, the Andersons' first live-action TV series, which also re-used many of the film's props.