A chronicle of astronaut Scott Kelly's 12-month stay on the International Space Station, which is laying the groundwork for a manned mission to Mars, as well as his training for the mission and return to Earth. The two-part documentary also features his identical-twin brother Mark Kelly, also an astronaut. Because of their shared DNA, NASA scientists are using Mark as a control subject to determine any physiological changes?down to the molecular level?that the long space stay has had on Scott.