I have no real knowledge of Thor other than the Marvel movies and some heavy metal songs that I kind of understand, so my background of Thor’s Marvel mythology is limited.
Thor, that chiseled Norse god of the uncovered torso, can’t get it up. He can’t pick up Mjolnir, his hammer. It’s just sitting on a moon. He’s been trying for weeks to pick it up and it won’t budge. Thor won’t eat or sleep. Thor just wants to hammer, but he can’t. Finally, he goes home to his parents’ house (Asgard castle).
While he’s away from the hammer, a blonde woman picks up Mjolnir! Who is this woman whom Mjolnir has found worthy? No one knows! 
What we do know from the mysterious woman’s inner dialogue bubbles is that she is courageous and just as confused as the reader about what is happening. She has the powers of Thor. The hammer, which is sentient and seems to communicate with her, responds to her will in ways it never responded to Thor Odinson. She uses her powers to fight Frost Monsters, corrupt oil executives, and other bad guys who need a good beating.
Unfortunately for the woman with the hammer, not everyone is very accepting of her new role. Old Thor, for one, doesn’t understand what happened to his hammer and why this woman stole it. Odin is just mad at everything and seems really upset that his wife did a good job running Asgard while he was gone. (Odin was gone from Asgard for some reason not explained in this particular comic.)
That is the basis of this comic – Thor lost his hammer, a woman picked it up, and everyone is confused.
The writing is very good. Jason Aaron mixes fantasy with humor and modern social commentary nearly seamlessly. I haven’t laughed out loud in a comic since Hellcat, but this comic made me chortle on the bus (Lady Thor’s inner monologue: “Oh, man. Oh, wow. Quick. Say something badass.“). I grew to like Lady Thor in less than a single issue. Thor is portrayed as a good-hearted doofus jock (watching him try and do detective work is great). The bad guys are deliciously smarmy.
Fun read. I recommend.