Season 3, Episode 9, "Slapstick"

WARNING: SPOILERS AHEAD! Prez accidentally shoots and kills a cop named Derrick Waggoner and feels really bad about it. McNulty goes out for late-night sex sessions in Washington D.C. and leaves his kids home alone, but Teresa D'Agostino eventually drops him when she finds out he is apolitical and didn't finish college. Avon is upset about his nephew's death but decides to suck it up in front of his sister. Cutty begins to break ground on his new venture, but not without going through a whole lot of bureaucratic red tape first. Omar and his grandmother almost fall victim to Stringer's violation of the Sunday truce, which convinces him he needs to finish the job with the Barksdales by himself. KimNulty enlists the help of Bubbles yet again. Carcetti learns that Gray is coming along in his mayoral candidacy and feels momentarily uncomfortable about going behind his back for his own political ambitions.