Sarah (Eva Green) is a French astronaut training at the European Space Agency in Cologne. The only woman in the arduous program, Sarah struggles with feelings of guilt over the limited time spent with her young daughter, Stella (Zélie Boulant-Lemesle). When Sarah is chosen to join the crew of a yearlong space mission called Proxima, it creates chaos in the mother-daughter relationship.

When Sarah is invited to join the crew of a yearlong space mission called Proxima, alongside veteran astronaut Mike Shanon (Matt Dillon), tensions escalate and her insecurities about motherhood rise dramatically to the surface.