He gets an email from Goldstone (a shady character involved in both Russian politics and the slimy music industry) saying the Russian government wants to help Trump by handing over some dirt on Clinton. Goldstone sets up a meeting with a Russian lawyer flying over from Moscow. Don Jr. assumes this must be the dirt he was told about. Except the lady wastes his time and has no dirt.

Was he set up?

By this time Hillary knew the DNC emails had walked out the door on a thumb drive. She knew their release was imminent along with likely more emails to come. Podesta had already proposed painting Trump as a Russian stooge as a good line of attack given his ties to the country. Was this an effort to entrap the family "Fredo"? Why else would Goldstone 1) feel him out 2) arrange the meeting 3) show up with an empty suitcase?