Batman breaks up a meteor that was going to crash into Gotham City and one of the chunks is carrying a spaceship that contains a blonde teenaged girl. She speaks an alien language, is super strong and shoots red lasers from her eyes. Batman manages to subdue her with a chunk of green Kryptonite. When Superman appears he recognizes her language and says that she is his cousin, Kara. She stays with Batman for a while and learns English, though she wants to be a human girl. Clark Kent takes her shopping. Then, Wonder Woman and other members of the Justice League show up and tell them that she's a threat and that she needs better training. So Wonder Woman takes her back to Paradise Island. But Darkseid of Apokolips wants this "SuperGirl" as a new warrior and so has her kidnapped by Granny Goodness. There are some casualties. Superman and Batman (voices of Tim Daly & Kevin Conroy) get the help of Big Barda, who had defected to Earth years ago. She leads them back to Apokolips and a confrontation with Darkseid and the corrupted Super Girl.
I have about the same reaction to this as I have the last few DC Comics Animated features. It' just "okay". The one amusing thing, though, is that Ed Asner does the voice of Granny Goodness. That made me laugh every time she talked.