The central columns of this episode are our visits to Tyrion's cell, as he tries to arrange a champion to fight for him in his trial by combat. Cersei's champion will be Ser Gregor Clegane, “The Mountain”, whom we're shown butchering a crowd of helpless peasants at the beginning of the show, presumably as a warm-up.
The first choice, his once-masterful brother Jaime, is now missing his dominant hand, and his efforts at training the other one haven't gone so well. The next choice his Bronn, whose victory in a previous trial by combat helped the two forge a lasting relationship– though neither would use that word. Bronn, though is all set to marry Lollys Stokeworth, and off her older sister Falyse so he'll inherit their father's castle– he doesn't fancy his chances against The Mountain, anyway. Tyrion applauds his selfishness, but looks doomed.
An unlikely ray of light arrives in the form of Prince Oberyn of House Martell. He believes The Mountain killed his sister, and sees the trial as a chance to extract justice, although Tyrion warns him that King's Landing is the entirely the wrong place for that. It's a noble move from a very likeable character; since Oberyn joined the cast at the beginning of this season he's been one of my favorites: cocksure of himself, philosophical about his licentiousness, and entirely unmoved by the machinery of power used by the ruling Lannister/Baratheon house.
Who will triumph? We know little about Oberyn's combat skills besides his reputation as a formidable warrior, which is pretty nebulous. If The Mountain excels in strength then we might reasonably suppose Oberyn excels in skill– there was a brief glimpse of the trial by combat in the season 4 trailer, and it looks spectacular.