Zor-El, Kara's father knows that Krypton is doomed. He has his own plans for his daughter which are not unlike his brother Jor-El's plans for his infant son Kal-El. A mysterious interloper though complicates Zor-El's actions and involve Alura (Kara's mother) in ways that she doesn't understand...
Effectively flushing out Kara Zor-El/Supergirl's origin story, Supergirl#0 continues the trend of decently strong storytelling that writers Green and Johnson have made a staple of The New 52's Supergirl. The best thing about their Supergirl origin tale is that it is straightforward without being devoid of tantalizing teasers and developments that will continue to unfold in the pages of the ongoing Supergirl monthly comic book. Plenty of questions are raised, but thankfully (unlike the early pre-New 52 incarnation of Kara) her motivations and those of her family are not suspect in the way that they were then. Zor-El wants what's best for his daughter, period.
Asrar's artwork is also solid as ever. Something that has also stabilized Supergirl in a way that the other Superman Family titles haven't been able to be stabilized (with the possible exception of Grant Morrison's Action Comics). I still hate that Kara's redesigned super-suit reveals way too much of this 16 year old's penchant for excellent bikini waxes, but at least Asrar isn't throwing Kara at us in porn star worthy poses.
Supergirl has been a pretty good book since its re-launch. If only DC Comics would allow the Superman Family titles to act like they should, i.e. a family of books. Superman is the type of character (and franchise) who needs strong supporting characters, and continually keeping Kara from becoming friends with her cousin, and settle down with a secret identity, a job, etc. is beginning to wear on the patience of this (and many other) Superman fans.