Supergirl 5x9

The Monitor sends Harbinger to gather the worlds’ greatest heroes - Supergirl, The Flash, Green Arrow, Batwoman, White Canary, The Atom and Superman - in preparation for the impending Crisis. With their worlds in imminent danger, the superheroes suit up for battle while J’onn and Alex recruit Lena to help them find a way to save the people of Earth-38.

This crossover continues on Batwoman S1E09, The Flash S6E09, Arrow S8E08, and Legends of Tomorrow S05E01.