Guess the Plot
1. Benjamin, a goliathus beetle growing up too quickly on the scalp of a squirrel monkey, is pinned between finger and opposing thumb and transported toward a row of gnashing teeth.
2. Emma Brown has been snatched... by Aliens. To save the lives of all the people on Earth, Emma must outplay creatures for whom cheating is a way of life, and win the Galaxy Go Fish Championship.
3. When your little sister's been snatched by shifters, mister, it's revenge time. Time to get your gun and take out every shifter you see. Assuming your sister finished cleaning your weapon before she was snatched, the brat.
4. Rogue government agents abduct Bill and spirit him to a secret lab. He awakes in a hospital, aware that he's been through major surgery, but strangely it is his emotions that feel most different. Only when Bill tries to use a bedpan does he realize that things are not as they used to be for
5. In the woods practicing her Wiccan rites, Helen is abducted by aliens. Seems the Grand Chief would like to add a witch to his harem. Although her powers are diminished when away from Earth, she isn't giving up without a fight. Now if she could only get to her wand.
6. For Erika, Monte Carlo's all sun, surf, and shopping on Daddy's credit cards until she's snatched from the lobby of her hotel. In the hands of international arms dealers, Erika learns just how Daddy's fortune was built. Can she use her martial arts training to save herself, or will she become yet another casualty of war?
Dear Agent X,
The shifters stole Kara’s parents away when she was ten, leaving her and Lizzy to fend for themselves. For kids underground? That meant signing up with the military. [When Kara was ten? And her sister even younger? (I looked ahead.) What good would they be to the military? No one's gonna want to lead a squad of seven- to ten-year-old girls in taking some hill.] [Wait, I know: Kara, Lizzy, there's an army of Ken dolls heading this way. We need you to lead your Barbies into battle.] At seventeen years old, all Kara’s got left is her dad’s cottage, a heightened sense of paranoia and her kid sister. Until she spots the shifter on her first surface patrol. [That word "until" suggests that spotting the shifter has somehow changed what Kara has left.] Slate gray eyes empty of emotion and spiderlike arms [Their arms are like spiders? If these are shapeshifters, I would expect them to have arms like swords or sledge hammers or cobras.] that would rip her apart in a heartbeat, it was everything she’d imagined in her nightmares. These were the monsters that pushed humanity underground and made them terrified to step topside. [Earlier you implied that being a kid underground had something to do with the need to join the military. Now you're basically saying everyone's underground. Get rid of "For kids underground?" ]
And once one shifter is sighted, there’s [there are] more to follow.
The sergeant drops Kara on the roster of unlucky soldiers heading to the surface. [She was already patrolling the surface.] All that honor stuff is crap, but the adults lay it on heavy anyway, trying to justify sending folks to their deaths. Have to protect the borders if they want to survive. Not so easy to break to Lizzy though, that she might not come back. She heads home, expecting to find her sister curled up reading a book and not cleaning her gun like the brat was supposed to. Kara walks into an empty house.
They took her folks. They took her best friend. Now, the shifters snatched Lizzy. Fighting them isn’t enough. Kara wants revenge. [Isn't fighting them the way to get revenge? Does she have something else in mind?
"Snatched" is an 84,000 word YA science fiction.
If humanity has been driven underground, how is it that Kara and Lizzie have a cottage? Do they have a white picket fence? It seems likely that if humanity were driven underground, they would put all their resources toward taking back the surface, where they can grow food, rather than settling in, building underground houses, etc.
Sounds like you've got a good tone for describing this plot, but it's a bit disorganized. Every sentence should follow logically from the previous sentence. A few of yours seem to just be tossed in at random. Tighten it up.