It's all to make an I-Don't-Like-You point, of course, since the one in charge is the one she sent to jail sometime I don't remember ago in the past.
So she's free, and pissed off and sporting a nice bump on the head the whole ep. Peter's concerned about her to the point where when she says "Who care about me?" (meaning there's more important things to worry about), he says "I care about you!". Olivia looked stunned, then awkward, Walter looked pleased as punch, and I went "Squeeeee!" so much that I had to back up and watch the next part over because I was distracted.
They're looking for the killer of two epidemiologists by way of two giant spiky slugs, who they've already established is the same person as her kidnapper thanks to the evidence she hid. The trail leads back to Agent Loeb-- the dude who had the heart-bug, then who got Crazy Brit out of the German jail by way of teleportation right before Liv was kidnapped-- and his wife, Meg March of Little Women fame. While she's breaking and entering and getting caught and pretending she's just being neighborly, Peter's being asked to be a criminal in the stead of the FBI who can't, and is getting wire taps. They pick up a call from Meg March to Hubby Heartworm where they decide to kill Olivia, and manage to warn her-- and it's MM who winds up dead, but HH doesn't know that, and they trick him into 'meeting' her. Liv uses the crime scene pics as a not so delicate way to break him so he'll confess, which he does, but says that he was trying to save her, that they were the only chance (for what, he didn't mention), and that she doesn't know what she's done.
He's hauled off, and Liv can finally relax, but what he said makes her look all lost-puppy, and Peter stands too close trying to get her to relax / let it go, and Walter keeps insisting that Peter was a little more worried than might be expected when she was missing. Which we didn't get to see.
Not a bad ep, but it seemed a little like a well-edited two-parter that was condensed into a one-parter for whatever reason. Lots of stuff going on, lots of movement, lots of tenseness, but not alot of time for reactions-- it would have been nice for a little emotional growth while she was missing, but she wasn't missing long. If she was checked out in the hospital, it wasn't mentioned. She went right back to work. You'd think that JJ could make it emotionally dense with only a few characters the way he did on lost, but I guess that's not what he's going for here. X-Files is what he's going for here, and he's got the questions and misdirections feeling down perfectly... just not the pacing, though it is improving as the season moves along. Maybe they just haven't quite settled their writing team yet.
And why did her sister move in with her? She's got to be a plot device waiting to happen. Like, she's being paid to spy on her or something. That'd be sweet.