Details about the actual plot in the episode have been kept pretty well under wraps for a while, but the logline for the double episode indicates that first, Jess will encounter Jake while visiting New York and will be sucked into helping out with a case by the detective.
Then in the New Girl part of the crossover, the Nine-Nine will interact with the rest of the cast, and the mystery of why Jess will be in New York will be unveiled.
Considering that Brooklyn Nine-Nine spent its early episodes this season featuring Jake Peralta and Captain Holt hiding out from some bad men who wanted them dead, this crossover episode should be a nice change of pace for the police comedy.