It's been almost a month since Nathanial and Annabelle rejoined HMAS Sovereign. For Annabelle it's been a journey of uncertainty; she had expected a happy reunion with George Bedford, first officer of the flagship of Her Majesty's Royal Navy, but instead he's been distant. She fears it has something to do with her newfound disability. For Nathanial, however, the journey has been uneventful since he's spent the entire time locked in the brig under the cloud of treason.

Things change abruptly when Sovereign is ordered to return to Luna, and retrieve Doctor Cyrus Grant, who has been sending increasingly confusing heliograph messages back to Earth. There is an air of uncertainty in Otterbein Base, and concern over Grant's well-being. Once again he's gone missing, turning his back on the Selenites and the British research team stationed there – leaving with creatures who are neither human nor Selenite.

A search-and-rescue mission is soon underway, taking our heroes deeper inside Luna than ever before. There they will discover the mysteries of the Drobates, and their amazing City of Light and Science. Annabelle is concerned that her uncle will no longer accept her, and Bedford is concerned that being on Luna once again will have adverse effects on his captain, but these things are the least of their worries. Grant is close to uncovering the answers to an age-old secret, but he is not the only one who seeks this knowledge. A creature stalks the dark underworld of Luna, a creature once human, and quite insane.

Dark Side of Luna is the series one finale of Space: 1889 & Beyond, and signals a new era of political upheaval and adventure for the series, as the key to humanity's future is unveiled.