A freak accident has taken the lives of six scientists on the Moon, leaving another astronaut stranded in orbit with no hope of rescue.
As she contemplates suicide, she intercepts a faint signal from the far side of the Moon, calling himself LUNA. He claims the deaths were no accident, and someone else is after him.
She only has 40 hours of oxygen left. With each pass around the Moon, she learns more about LUNA, and with every revolution, more attached as she realizes their lives are intertwined.
But she doesn't know who to trust, and time is running out.
After all, if no one else is alive on the Moon, doesn't that make him...the killer?