"A man with no motive is a man no one suspects."
Jaime visits Tyrion in the Black Cells after Bronn chastises him with a story: Bronn only stood for Tyrion at the Eyrie because his first choice, the Kingslayer, wouldn't have made it in time. Jaime confirms his brother's suspicions about Cersei - she wants Tyrion dead. Tyrion asserts his innocence, but Jaime can do little to help him or Sansa, whom Cersei is convinced also played a part in Joffrey's murder. When the queen regent learns of Jaime's visit to the dungeon, she summons her twin and questions his loyalties.
Aboard Littlefinger's ship, Sansa finds out it's headed to the Eyrie. Confronted by Sansa, Littlefinger confesses he had a hand in Joffrey's death. He reveals to Sansa she played a part as well - the "heirloom" Ser Dontos gave her contained the poison that killed the king.
In the Red Keep, Margaery is shocked to learn Olenna Tyrell orchestrated the assassination to save her granddaughter from marrying Joffrey. Olenna shares a narrow miss of her own: Originally engaged to a "twitchy" Targaryen, she changed her destiny by wandering into Luthor Tyrell's chambers and stole away her sister's fiancé. Now it's Margaery's turn to "do what needs to be done," Olenna states. That night, Margaery pays a late night visit to Tommen, who is dazzled by his intended.