Promising American astronaut Trish Mulligan is part of a mission to scout the moon for potential base locations. Her ship is not supposed to land, but an onboard accident forces her to maneuver a difficult emergency landing that results in the death of her crewmates and the destruction of their ship. Stranded alone on the moon, Trish hikes to the top of a mountain in order to make radio contact with Earth. She learns that a rescue mission won't be possible for another 30 days. Shortly thereafter, the radio transmission cuts out. Trish's high-tech space suit contains solar panels that power her oxygen supply. The sun will set in three days time, and night lasts on the moon for 14 days. Trish cannot go that long without sunlight, or she will quickly run out of oxygen. With no form of shelter or supplies, Trish realizes that her only chance of surviving the next month is to chase the sunset, walking continuously across the lunar surface in order to lengthen her time in the sun. Trish sets off on her journey, rarely pausing to rest. Over time, her feet grow blistered and she begins to have visions of her dead older sister, Karen. Trish talks to Karen about their relationship, while traversing the increasingly difficult terrain. A malfunction in her suit slows her down, and the darkness starts to catch up to her. After a month, Trish realizes that she has veered off course. She cannot find her ship or the mountain where she called for help. After airing her grievances with Karen, Trish agrees to say goodbye and let her sister go. As darkness closes in, Trish hikes to the top of the nearest mountain to catch the last rays of sunlight available. Meanwhile, the rescue mission arrives at the site of Trish's abandoned ship. They assume that Trish chose to spend the end of her life exploring the moon, and they wonder how far she got before dying. Because of Trish's altitude, she is able to make a weak connection with the crew, letting them know that she's alive. They are shocked to learn that she walked the entire circumference of the moon. They rescue Trish, and she describes the incredible places she saw, insisting that she will one day return to continue exploring. As the rescue ship lifts off the moon, Trish thinks she sees a lone figure standing on the surface, waving goodbye. But when she looks again, there's nothing out there.