Oh this is good.
Arianne was last seen trying to cross a river with Myrcella and her conspirators. Captain Aero Hotah captured them. Kingsguard Ser Arys Oakheart threw himself at Hotah's axe and died, presumably to save Arianne whom he loved since she seduced him into helping her. In the confusion, the mysterious Darkstar tried to kill Myrcella, which he thought would bring on war with the Lannisters more quickly. Her horse moved away just in time, saving her life, but she lost an ear and her face will be scarred.
Arianne is locked in a tower for several days or weeks. She broods about the uselessness and cruelty of her father, Doran Martell. He's the typical awful father that important lords make, having to be more concerned about the security of the realm than the happiness of their daughters. He tried to marry her to a succession of awful old men. He kept her coddled and out of state affairs. She was the rightful heir to the throne of Dorne but she spied a letter to her brother Quentyn promising him the kingdom instead of Arianne. Even now Quentyn is across the narrow sea hiring The Golden Company so he can take Dorne from her.
Except that almost none of that is true. Doran Martell is not the meek and useless man everyone thinks he is. He's playing a longer game, is all. Ever since the Lannisters killed his sister Elia and her children, he has been plotting revenge by marrying his family with the remaining Targaryens. The letter was to Viserys. He would marry Arianne to him and Viserys would, through her, inherit Dorne. He didn't tell Arianne about it because she is a blabbermouth, and the parade of old men was deliberate: he had to make it look like he was trying to marry her off but knew she would refuse them. Drogo put paid to that plan when he poured gold over Viserys' head all the way back in book one. So now Quentyn is off trying to find Dany and marry her, and bring her and her dragons back to finish off the Lannisters. I'm even wondering if he had something to do with Dany getting those dragon eggs.
The trouble now is that Arianne's foolishness means that when Kingsguard Ser Balon Swann turns up with GREGOR's head, he's going to find out what happened to Myrcella and war with the Lannisters might come too soon.
