The movie opens twice - the first time in 1989, with Hank Pym (a digitally youth-ified Michael Douglas) quitting S.H.I.E.L.D. over Stark senior's desire to weaponize Pym's shrinking technology. The second time is after the title sequence and set in the current day, with Scott Lang (Paul Rudd - an unlikely but reasonably successful action hero) getting out of jail after a stint for burglary. He's picked up - and housed - by his former cell-mate Luis (Michael Peña - who has a large role but went over-the-top anyway ... although he's reasonably charming and funny). It's quickly established that Lang has a Masters in Electrical Engineering (I approve), his most famous heist was both technologically extremely difficult and charitable not lucrative (giving money back to people who had been ripped off), and that he has a young daughter he desperately loves but can't see much because he's an ex-con (and her would-be step-dad is a cop). All of that in about ten minutes, it definitely felt like a data dump. A reasonably well constructed one, but so laden with information that it screamed "prep!" as it happened. And then Hank Pym is back, fighting his former associate Darren Cross (Corey Stoll) who's nearly recreated the Ant-Man tech and is clearly evil because he wants to weaponize it.
All of the characters are drawn a little too broadly, but the actors are also clearly enjoying themselves in a way that makes the movie entertaining to the viewer. The end result is flawed, but fairly well constructed and a lot of fun.
Like any Marvel film, there's both a mid-credits scene and a post-credits scene to look for. The mid-credits scene directly relates to the movie you've just seen, and is fairly rewarding in context. The post-credits scene is an idiotic hook for "Captain America: Civil War," which tells you essentially nothing if you haven't read the related comic books, and hardly anything even if you have.