Crisis on Earth-X means to watch the following episodes as part of the crossover or a.k.a. "Arrowverse":
Supergirl S03E08 Crisis on Earth X (I)
Arrow S06E08 Crisis on Earth-X (II)
The Flash S04E08 Crisis on Earth-X (III)
DC's Legends of Tomorrow S03E08 Crisis on Earth X (IV)
Gina the best civilian ever.
Enjoyed the episode and went as expected.
