Looking for help, Dumbledore will turn to a former student who we know Dumbledore thinks quite highly of, Newt Scamander.
Scamander, in turn, will get help from the returning Queenie and Tina Goldstein as well as his no-maj friend Jacob Kowalski.
New details revealed on Pottermore specifically explain that as the sequel opens Grindelwald will escape, forcing one Albus Dumbledore to get involved in the efforts for his recapture.