The Pierce family is scared for Jennifer's life.

(This is technically part of the "Crisis On Infinite Earths" crossover. Crisis begins with Supergirl 5x09, but this episode leads into it).

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.

The group uses Ray's invention to track new recruits to help save the universe. The Monitor sends Iris, Clark and Lois in search of a mysterious Kryptonian, while Kate and Kara head out to find Bruce Wayne. In addition, Mia challenges Sara, Rory discovers a hidden talent, and Lex Luthor returns.

Pariah enlists Black Lightning to help stop the Anti-Monitor after the Flash of Earth-90 shares what he learned from his battle in 'Elseworlds'. With the help of Black Lightning, Barry, Cisco and Killer Frost come up with a plan that could save them all. Meanwhile, Iris has a heart-to-heart with Ryan Choi while Oliver and Diggle return to an old familiar stomping ground.

Stuck in the Vanishing Point, the Paragons search for a way to escape. Oliver reveals that he has become something else. The origin stories for The Monitor and Anti-Monitor are revealed.

Worlds lived, worlds died. Nothing will ever be the same.

My thoughts about the crossover and your comments and questions as well.