When young hot-shot District Attorney becomes Gotham's white knight in the wake of the Batman's assault on crime; the the mafia turn in desperation to the chaos loving 'Joker' who offers to cleanse the city of the symbols they believe in and prove that whatever doesn't kill you simply makes you… Stranger.

Tad long for a logline. Perhaps it’d work nicely as a blurb.
The last part ‘whatever doesn’t kill simply makes you stranger’ seems a little chucked in and not entirely relevant for a logline, but would work as a tagline. It’s also important that you mention what’s at stake for Bats, his love interest Rachel.