As the mission launches, Emma finds her mettle as commander tested by an onboard accident, a divided crew and a family emergency back on Earth.

Emma puts herself in harm's way when Ram unwittingly endangers the lives of everyone on the Atlas. Matt must make a wrenching decision about his future.

As Christmas approaches, the crew prepares to make their final calls home before communication grows spotty. Matt and Lex adjust to their new normal.

Emma's behavior becomes a cause for concern, Kwesi and Lu team up on a plant mystery, and Lex makes an impulsive decision with far-reaching consequences.

With only hours to go before Atlas' planned touchdown on Mars, Emma is confronted with everything that could go wrong, while Lu receives new orders.