The Prime Directive refers to Otto's mission to ensure the safety and delivery of the human crew aboard the Solarian to their destination.
As the Solarian's Liaison robot, Otto was tasked with monitoring the vitals of the human crew in Systema's Sleep-Hyperstasis, maintaining course to the mission destination, and performing general upkeep on the Solarian.
Because of an unexpected incident, Otto's prime directive was interrupted for a considerable length of time. Missing his memory component, Otto is unsure of what took place. He must piece together the events of the incident while trying to reverse its effects to get the Solarian back on course to its initial destination.