The Bat in the Belfry

After stopping a heist by Rupert Thorne, Batman returns to the batcave to learn that all criminals in Arkham Asylum have mysteriously been released. Batman finds out that it's a crazed clown named Joker whom released them, and he plans to make a blimp full of laughing gas to explode over Gotham so every citizen turns into a lunatic. It's up to Batman to stop him.