Following the harrowing events of the Errand of Vengeance trilogy, tensions between the Federation and the Klingon Empire are the highest they have been for twenty-five years. Even as Federation Ambassador Robert Fox engages in tense negotiations with his Klingon counterpart to maintain the peace, Captain James T. Kirk of the Starship Enterprise sees his vessel refitted from a ship of exploration to a ship of war.

The impact of the coming conflict will be felt by many of the Enterprise crew - and by those on the other side. Amongst them is security supervisor Leslie Parrish, who is stunned to learn she is carrying her dead lover's child - and that he was a Klingon spy, surgically altered to pass as human and infiltrate Kirk's ship. Meanwhile Karel, the brother of the Klingon agent who was posing as John Anderson, faces a crisis of faith and conscience as he sees the Klingon Empire falling into dishonourable ruin… And Michael Fuller, whose son Sam was killed in a battle with Klingons, reenlists in Starfleet. But is he driven by loyalty or revenge?