When four starships go missing without a trace, the Federation calls on Nick to investigate the disappearances. After a brief study of the available data, Nick learns that one passenger-a mole-was on board all the missing ships, which suggests the ships may have been attacked by pirates, or someone posing as pirates. Nick is aware that slavery and human trafficking are rife in the Sirian system, and suspects that Sirius is at the heart of the mystery.

When the same mole books passage on a fifth starship, so does Nick. If he can identify who is behind the disappearances, he might be able to gain enough information for the Federation to take them down; failing that, perhaps he can at least protect the passengers on the fifth ship.

It's a dangerous assignment with little chance of success, but someone has to try. The biggest risk to Nick is the fact that he is a wanted man on Sirius. For six years the Sirian KK has been hoping to capture him and punish him for his actions in Sirian Summer; if he ever falls into their hands, he is almost certainly looking at torture and execution, but if he doesn't take the risk, thousands of innocent people may be killed or enslaved.

Return to Sirius will be Nick's deadliest mission yet as his life comes full circle and he slams head-on into the most brutal regime in the galaxy.