Here's a plot summary:
The Superfriends are able to defeat Lex and Leviathan but Kara dies of kryptonite poisoning in the process (let's say her anti-kryptonite suit got destroyed in the fight somehow). They all grieve for her but Lena has a special kind of grief - she and Kara had only accepted their feelings & become a couple a few weeks beforehand.
In an attempt to distract herself, she buries herself in work.
A few months pass and the others slowly seem to be able to move on. Lena can't. And it ends with a lot of arguments in which she devastatingly finds out they had all secretly been blaming her for Kara's death. Since Lena had, unknowingly, enabled Lex to come so far in the first place. The contact breaks and they turn away from her.
Now alone with her sadness and thoughts, she comes up with a plan. She wants to find a different earth. An earth where everyone and everything is the same except for a few special criteria: Kara must be alive. And Lena must be dead.
But all her attempts to find one fails. Lena loses hope.
But at last she succeeds and finds an earth which meets the needed criteria. She and the Kara from that Earth (who has been mourning her Lena just as much as she had been mourning her Kara) find together and are both relieved to have each other again.
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