In the world of this novel there are seven kingdoms. Some are ruled by kind and wise kings and others by those who are neither kind, nor wise.
Among the peoples of the seven kingdoms are a few born with extraordinary talents, called Graces. Some are weather forecasters, some are exceptional swimmers, some are inordinately talented swordsmen, some can read minds and some are thought to be killers. A Graceling can be identified by their eyes, each a different color. Gracelings are often feared and shunned by the people.
The kings of the seven kingdoms call the Gracelings to their courts to use them as they see fit. If a Graceling has a talent the king finds useful, he commands their service.
Katsa was just such a Graceling. She was a skilled fighter and could disarm / kill hundreds of men in a single fight. Randa, her king, used her to put fear in the hearts of his people and to unfairly harm them or kill them.
Katsa's often could not control her Grace. She was quick to anger and often would strike out harder than she intended.
Katsa also rebelled against the unjust use of her power commanded by King Randa. With a few friends she formed a secret council which would go on errands of mercy within the seven kingdoms, righting wrongs. It was on one of these errands she first encountered Po, the seventh son of the king of Lienid, also a Graceling with skills similar to hers but not as severe.
With Lienid she travels to Monsea to discover who caused Po's aged grandfather to be kidnapped (Katsa and the council had rescued him.
Her relationship with deepens and she learns to control her Grace.
This is really a page turner. 471 pages which I read in 24 hours.