Doppelganger (1969)

The Americans may have reached the Moon, but the Europeans have also made quite a find. One of their unmanned probes has photographed a previously unknown terrestrial planet, orbiting the Sun exactly opposite Earth. This raises more questions than it answers, so the European Space Exploration Council wants to send a manned spaceship to investigate.

They don't have the resources to launch such a mission any time soon, but the clock is tickng. There's been a security lapse, and a hostile foreign power just might know more than we'd like. So they team up with the Americans, and hastily put together an expensive joint mission sending veteran American astronaut Glen Ross and British astrophysicist John Kane on a journey to the new planet on the far side of the Sun.

The journey takes three weeks, which they spend sedated on automated life support. They wake up entering orbit around the new planet, and take a landing shuttle down to investigate. But their landing shuttle crash lands in Mongolia, and only three weeks after they'd launched! A round trip in under six weeks should have been an astrophysical impossibility!

Back at EuroSEC, Ross insists they did not turn back, but left their spaceship in orbit around the new planet...and he sticks to his story under some harsh interrogation. Everyone is puzzled and Jason Webb, EuroSEC director, is desperate to keep the mishap a secret until he can figure out what went wrong and how he can get off the hook for the failure of this really pricey mission.

Ross quickly angers his wife - their marriage was already on the rocks - by insisting that everything is backward. Webb hears of this, interrogates Ross yet again, and discovers he can read normal text perfectly when it's reflected in a mirror! Kane dies from injuries sustained in the crash - or was it from Jason screaming maniacally at the poor man - and autopsy reveals his organs are on opposite sides of his body from where they should be.

Ross speculates he is actually on a mirror Earth, and that the two Earths are somehow connected, so that they remain exact mirror images of each other...and that his mirror image Ross - his doppleganger - is at that moment on the other Earth, thinking the same thing. Webb - or his doppleganger - agrees, and has a new shuttle built to send Ross back to the spaceship, still in orbit, to return to his original Earth.

But they have to guess whether electrical polarities are also reversed, and if they guess wrong, the new shuttle will be unable to dock with the spaceship. They guess wrong. The shuttle goes out of control when it tries to dock and the polarities are incompatible, and it crashes right into EuroSEC HQ, killing everyone but Jason Webb.

We then see Jason in an infirmary, looking decades older, telling this tale to a skeptical and dismissive attendant. He'd kept it a secret, and the few who'd known anything had been killed. Why should anyone believe him? He sees a reflection of himself in a mirror, propels his wheelchair toward it, and we hear the sound of breaking glass before the credits roll. So did it really happen, or was it all a grandiose delusion of a dementia patient obsessed with mirrors? Either way, it's a pretty grim ending.