Joker is madly in love with Harley.
At the time of the film, Harley is still imprisoned while Joker is still roaming free. There are a lot of flashbacks in the film and that's where most of the Joker shots are from. There is a flashback showing how Harley and Joker met, how Harley got caught (chase scene with Batman on the roof), and one about them jumping into a vat of chemicals.
Eventually, in a ridiculous scene, a prison guard slips a giant cellphone to Harley and she starts texting with Joker while she's doing the mission with the other squad members.
The film ends with Joker breaking into Arkham to rescue Harley.
Most of that teaser is in the film if I recall correctly.