The plot details and images have been released for "It's a Super Life," the thirteenth episode of season 5 of Supergirl, and more significantly, episode number 100 in general, which will see the return of Mister Mxyzptlk.
In case you have forgotten, the character is an almost omnipotent being of the 5th Dimension that can deform reality in a way that is magical for all intents and purposes. He appeared for the first time in the season 2 episode "Mr. and Mrs. Mxyzptlk", where he suddenly materializes to insist enough that Kara marry him ("Once you've been worshiped by the almighty Mxy, no there is a turning back ") and creates havoc after she continually rejects it. He was eventually defeated by Kara using his main weakness against him, which if he is tricked into speaking or writing his name backwards, he can be banished.
The plot of this new episode implies that the trickster reappears for reasons currently unclear to offer Kara the opportunity to go back in time and tell Lena her identity before Lex had the opportunity, allowing them to remain friends at the expense to alter history. Presumably, she will try this at several points in the past only for each attempt to have different disastrous consequences, which will eventually lead her to realize that the status quo must be maintained, no matter how much it hurts.
Little context can be deduced from the photos, but the somewhat chaotic appearance of some of the scenes reinforces the likelihood of temporary manipulation not going well, which Kara and super friends will have to deal with.
Milestone episodes are often used as an excuse to look back and take stock of a series, and this will not be an exception, as it uses the settings to revisit the key moments of the entire execution of Supergirl, bring back some familiar faces and celebrate the ways in which it has changed and improved since its inception.