Erin McKillen is seated at a table in Wayne Manor, with Bruce telling her that Matches Malone ransomed her off to the highest bidder, and knowing that they shared a past, "contacted" Bruce who "saved" her from being auctioned to one of the families who may want her dead. Erin doesn't take to Bruce's form of hospitality that well, and once she's untied, hits him, and asks if she can leave, calling her cousin to pick her up.
While Erin waits for her cousin, Bruce finds her in the manor and the two begin to talk about Shannon, Erin's deceased twin, who Bruce admits to being pretty fond of when they were children. And here we learn what happened.
After Harvey put Erin and Shannon away, and after two failed appeals, the McKillen sisters come up with a pretty morbid plan on how to escape prison, but the catch is, only one of them gets out. So they draw straws, and Shannon "wins." She tells her sister that she loves her, then proceeds to hang herself in her cell. Erin calls over the guard who was seemingly aware of the plan, as it seemed that for her cooperation, it was promised that her son would get a life saving operation, presumably from an organ donation. With that, the guard calls over the medical staff, and the plan goes into action from there. For legal reasons, Erin needs to confirm her sisters body, but when the doctor leaves Erin and the guard alone, some body switching takes place, and Shannon's body is placed in Erin's cell, as Erin is taken out of the building by some medics who believe they're handling a corpse.
Back at Wayne Manor, Erin's cousin shows up, leaving Bruce to share a few words with his old friend before she leaves. Once Erin is gone, Bruce knows that his work isn't done, and suits up, believing he'll be able to catch both Erin and Two-Face under one net.
Long story short, Erin's cousin betrays her, drugs her, and brings her to Gilda Dent's grave, where Two-Face is waiting. Turns out the crime families would much rather have Erin's cousin in charge, than Erin, who is never in Gotham and a bit erratic. Before Two-Face can get his revenge, Erin decides to spill the beans that once he kills her, the crime families are just going to kill him, as they'd discussed prior. This doesn't please Two-Face, and when Two-Face isn't pleased, bodies drop.
As Two-Face opens fire on the crime families that have surrounded him (lighting up Erin's cousin, specifically), Batman shows up, and starts fighting off the crime families as well. Two-Face "thanks" Batman for the save, but causing a giant grave statue to collapse on him, leaving him trapped in a dug out grave, where Two-Face intends to leave him.