After Iris goes missing, Team Flash work around the clock to locate her. Barry stops responding to Kara's message and the Kryptonian grows worried. She travels over to Earth-1 in order to discover the truth. Finding out Iris is missing, Kara volunteers to stay on Earth-1 and find Iris before it's too late. Elsewhere in the darkness, a Speedster and Kryptonian unite to destroy their enemies. The Flash, and Supergirl have defeated every threat that's come their way, but on the darkest of days, sometimes even the greatest heroes fall. [UPDATED EVERY WEDNESDAY & SUNDAY - SEQUEL TO 'HEROES JOIN FORCES'] Thanks to @meekayylaa for editing, proof reading, and putting up with my constant rambling about Crisis On Earth-1! She's been a major help, and without her I doubt we'd be here right now!