The Day After Tomorrow (also known as Into Infinity in the United Kingdom) is a 1975 British science-fiction television special produced by Gerry Anderson between the two series of Space: 1999. Written by Johnny Byrne and directed by Charles Crichton, it stars Brian Blessed, Joanna Dunham, Nick Tate, Katharine Levy and Martin Lev, with narration by Ed Bishop. Set in a future where environmental damage on Earth threatens the survival of humanity, The Day After Tomorrow follows the interstellar mission of Altares, a science vessel that uses photon energy to travel at the speed of light. After leaving the Solar System and reaching Alpha Centauri, their primary destination, the crew of Altares push deeper into space; there, they encounter phenomena including a meteor shower, a red giant and, finally, a black hole, which pulls the ship into another universe.

Originally commissioned to produce a child-friendly introduction to Albert Einstein's theory of special relativity, Anderson and Byrne conceived The Day After Tomorrow as a pilot for a TV series, with the episode title "Into Infinity". Anderson was ultimately unable to raise funding for additional episodes, making the pilot a standalone special. With a cast and crew that included veterans of earlier Anderson productions, The Day After Tomorrow was filmed between July and September 1975; this comprised 10 days' principal photography and six weeks' special effects shooting. The visual style of Space: 1999 provided inspiration to both effects artist Martin Bower, who built the scale models, and production designer Reg Hill, who recycled sets from Space: 1999 to create the Altares interiors. The theme music was composed by newcomer Derek Wadsworth.

The Day After Tomorrow was first broadcast in the United States in December 1975 as the third episode of NBC's after-school series Special Treat. In the UK, it was transmitted on BBC1 as a special, first in December 1976 and again in December 1977; a re-edited version aired on BBC Four in November 2014. Critical responses to The Day After Tomorrow have been mixed: while its model effects and music have been praised, it has divided opinion with its "psychedelic" images, which have been compared to the visual style of film director Stanley Kubrick. In addition, the story has been criticised for lacking suspense due to the fact that it was devised primarily to educate, rather than entertain. A novelisation by Douglas R. Mason was cancelled after the planned TV series failed to appear; a new novelisation by Gregory L. Norris was published in 2017.