Preceded by the kidnapping of Van Criss, Joker, Jonny Frost, and Panda Man hijacked a Chinook, bringing Van Criss along so they could force him to deactivate Harley's nano-bomb implant. The ultimate goal, of course, was to free her from Amanda Waller's custody while also preventing the explosive from later killing Harley.
To be added
The helicopter was shot down, killing two pilots, Jonny Frost, Panda Man, and Van Criss. Joker and Harley were the only two to make it out of the helicopter before it crashed, burning any possible survivors inside.