Joker came through your prison, baring guns and goons, ready to bust you out of prison. Let's back up. The Bat had chased you and Joker down, and Joker stupidly drove off a bridge in hopes of losing Batman. He hadn't taken into consideration you couldn't swim, so by the time Batman dived in the water to go see if you or him were still in the car, he found you busted through the windshield, laying across the roof of the car.

So that's how you're in prison. Because of Batman and Joker's stupidity. Also, why Joker had you tracked down. It had been nine months since you'd seen Joker or his goons.

And that's exactly why the Joker, your boyfriend, has him and his goons dressed as guards as they shoot down both actual officers guarding your cell, sawing off the technical lock on your cell. The Joker entered, not making his actual appearance until he took off the visor, dimming his face and making it unable hard to see who he was. He tossed the visor, opening his arms for you.

"Bring it in, my sweet." He growled, looking at you with that grin.