Astronaut Emma Green prepares to lead an international crew on humankind's inaugural mission to Mars, but is haunted by an incident that shook the first leg of their journey -- and the thought of spending three years away from her husband and daughter.

The mission to Mars hits another snag after the Atlas solar panels fail to deploy properly, requiring Emma and Misha to make a space walk to repair them. Matt learns that his road to recovery will be longer than he hoped.

Dr. Wang Lu is grieved to learn that Mei Chen, with whom she has a secret relationship, has been reassigned away from Mission Control. While Emma wrestles with crew tensions, Alexis returns to school with her own uncertainties.

The Atlas worries about a widespread infection when Ram falls gravely ill, hallucinating about his childhood and a personal tragedy. Matt rethinks whether he should go home.

After Matt returns home he and Alexis face the holidays without Emma. As the crew prepares for their final call home before going out of range, Misha's eyesight begins to show signs of failing.

The crew must trust in Misha and his failing eyesight to repair the ship's failing water system. Kwesi remembers his childhood, and a moment of distraction leads to Matt's first meeting with Isaac.

Emma feels the strain of the increasing distance from her daughter, who is growing closer to Isaac. As her state of mind deteriorates, her family and colleagues grow concerned.

The Atlas crew and Mission Control anxiously await a signal from the Pegasus rocket, meant to land on Mars with the supplies needed to save their lives. Lexi looks for answers, and Emma faces the prospect of a failed mission.

Two weeks out from Mars the Atlas water system fails, forcing the crew into a high-risk operation to drill for the water lining the ship's hull. Lex reconciles with Isaac.

As the crew prepares for their landing on the Mars surface, Matt and Alexis and the other families gather to watch from Earth. Lu receives instructions for her big photo op, and Lex gets the results of her test.