Upon investigating a raided Federation outpost, the Enterprise is drawn into a long-standing clan war among the Acamarian people. One faction, The Gathers, who were responsible for the raid, have exiled themselves from their home planet, refusing to take part in the others' treaty(s) ending the clan wars. Picard convinces the Acamarian Sovereign to once again seek reconciliation. The Sovereign brings her personal servant, Yuta, who secretly attacks one of the Gathers at the first peace meeting. At first it appears as if the man died of natural causes, but Dr. Crusher soon discovers that it was actually a microvirus that had to have been genetically engineered to target a certain Acamarian clan. Data and then discovers that it is Yuta who carries the virus, and Riker rushes to stop her before she can kill The Gathers' leader. He arrives at the peace talks just in time, but must kill Yuta, whom he has been infatuated with. The Acamarians are grateful and go on their way. Riker is put out by the whole affair.
Though, seriously, that woman should give it up. Maybe I just wasn't raised with enough feuding instructions, but I find Yuta's character to be a prime example of why people should grow old and die at a rate proportionate to their fellow human beings. Otherwise you'd have those die-hards (pun intended) who would be carrying on prejudices and the like for eternity, because their papa raised them that way. Think about it - if people didn't die out in their natural time, we'd probably still have segregated buses (or have them again). Note that this is also why cryogenically frozen Klingon soldiers are a bad idea. Yuta might as well have been frozen all this time for as much as she was willing to change her character and sense of purpose.
Poor Riker. They might have been good together if it wasn't for the whole obsessive assassination thing.