Sterling Gates has done an excellent job on "Supergirl" so far, bringing the character's all-over-the-map behavior into something resembling a clear focus and integrating Kara Zor-El into the Geoff Johns Superman saga with seamless efficiency, but what's really compelling about "Supergirl" #39 is the mystery of Superwoman. What's her deal, and why is she so damn nasty?

I've always enjoyed Elliot S. Maggin's version of Superwoman -- Kristin Wells, scholar from the future -- who first appeared in "Miracle Monday," an essential part of the Maggin Superman canon. Gates's Superwoman doesn't seem to have any connection to the Maggin character, but her costume design recalls that of Kristen Wells, and that visual link helps provide a sharp contrast between the characters as this new one acts like a hero at first, but, as shown in issue #39, she's a vicious murderer.

Or is she really? We see her use her heat vision to blow up an innocent civilian's apartment -- with the civilian still inside -- but everything about Superwoman is a mystery. She's not even Kryptonian, apparently, and we don't know what her ultimate plan is. Gates has done a nice job setting Superwoman up as a possible mentor and ally of Supergirl, only to have her turn out to be a stone cold killer. But why is she doing what she's doing? We don't know, and Gates has turned it into a compelling mystery.

There are a few false notes in this issue, but none of them really have to do with the overall Who-or-what-is-Superwoman plot. Inspector Henderson writes a lengthy journal entry in "Supergirl" #39, and while he might consider himself a philosopher/cop of some sort, some of his prose is a bit too flowery. As a running joke, it might work -- here's this tough guy police officer who thinks he's Thomas de Quincey when he sits down to log in the day's events -- but it doesn't seem like it's supposed to be presented that way.

And the biggest fault of issue #39 is the brief fill-in art of Talent Caldwell. Regular artist Jamal Igle handles most of the pages here, but Caldwell comes in during the middle of the issue and draws a few pages in his J. Scott Campbell-influenced style, and it's just a completely jarring change from Igle's more classically-oriented approach. Fill-in artists are not unusual in DC comics, but the contrast in style hurts the issue. It's like having completely different actors play the main characters for 20 minutes in the middle of a movie. This is a Supergirl comic, not a Luis Bunuel film, and the pouty-lipped female characters of Caldwell don't add anything but a distraction to the story.

The issue ends with a few nice moments -- and the return of Jamal Igle. Supergirl's visit to the police station results in a funny bit of dialogue: "We can't walk her through the building dressed like that," which has interesting layers of meaning. First of all, she's a Kryptonian, and she can't be seen dressed in her Kryptonian superhero costume anymore because her people are banned from Earth in current continuity. But second, her miniskirt and belly shirt are ridiculously inappropriate garb for anyone -- a holdover from her more "extreme" days -- and getting her to cover herself up to interact with normal humans is a wink to the reader. Plus, having her draped in an overcoat for a scene allows her to rip it open and reveal the S-symbol before flying into action. And who doesn't want to see things like that in a Superman Family comic?

"Supergirl" #39 is a worthy addition to the overall Geoff Johns-driven Superman mega-story, and Gates propels the Superwoman mystery along nicely. Now that "Action Comics" and "Superman" have turned their attention to the third-stringers of Metropolis, it's nice to have "Supergirl" maintaining its newfound consistency as a comic worth reading.