The threat from the JPU has been eliminated and the empires should be at peace, but fractions within the SAS have revised old resentments against the Riss and Freeland. These anti-Riss groups have distorted the facts of the war with the JPU and dispute the Riss's right to quarantine JPU, insisting the SAS should have exclusive rights to the JPU and the Riss quarantined.
While the SAS plots against the Riss, an alien force enters SAS space and is annihilating inhabitable systems. The Riss attempt to form an alliance with the SAS and UFN to stop the alien invasion. But the threat from aliens isn't enough to avoid dissention and rebellion within the SAS.
Meanwhile, newly discovered humans in a neighboring section of the galaxy are at war with other aliens—close enough to eventually be a threat. The Riss must choose to remain neutral or get involved.
NOTE: Book VI in the Riss Series, The Riss Enemies, can be read as a standalone novel although it contains references to events in previous books.
Book V, The Riss Challenge, can also be read as a standalone although it contains references to events in previous books. It explains the war with the JPU and the reason for the quarantine.
Book III and IV, The Riss Survival and The Riss Accession, are best read together. They cover the invasion by an advance race of aliens using robots as hosts.
Books I and II, The Riss Gamble and The Riss Proposal, are best read together. They introduce the Riss, Gorilla-looking but peaceful aliens, and details the human protagonist's rise to spokesperson and leader of the Riss Nation.