Complex sentence: Once Batman is back, he finds out that all of the members of the Justice League are under attack by the plans he himself had created, and one by one saves them all, and tells them about what had happened.
Split into simpler sentences: Once Batman is back, he finds out that all of the members of The Justice League are under attack by the plans he himself had created. Knowing ways how to save them, he one by one saves them all, and tells them about what had happened.