Many stories begin with the hero or heroine already up a tree and they are extremely content up there. They have a beautiful tree house and an extraordinary tree job. They share the tree with a loving family or friends.
The writer comes along and throws something at our heroine, Frances, plummeting her to the ground.
Fatality instantly comes to mind, though the fatality doesn't necessarily mean the death of someone. It could be a Fatal Attraction to someone who lives in a neighboring tree or a Fatal belief in something Futile or Frighteningly False.
There was once a story about a young heroine and her menagerie. The writer threw Flying Monkeys at her. Unfortunately, two stories with flying monkeys might be a bit much. You can name a bar the Flying Monkeys' Saloon, or a fraternity, the Flying Monkeys Fellowship but the doctor said, "No more monkeys jumping
Fire is bad whether one is up the tree or down, unless one is stranded on an uninhabited island after Falling from a Freighter. Hopefully, the Freighter was closer to Fuji than Finland! Rescue would absolutely not be good if Frances, our heroine, is Frozen when she is Found.
Speaking of Found! What if Frances was hooked up with her fantasy tree house by Witness Protection (her name is really Felicia) and the Fiend fumbles upon her? What if she ran away from a personal Fiend, not a Felon fiend, so she is just hiding on a remote branch of the tree on her own; no Witness Protection program Facilitation. Wow, she would really be out on a limb! (sorry, couldn't resist)
Flashbacks! Yeah, that's the ticket. Frances (or is it Felicia?) is remembering snippets Fraught with Fear. Will she ever Fit the pieces together?
Fourth Dimension: Wow, that could be a whole new assortment of F-words.
Foreign Body: Medically speaking a foreign body is something that doesn't belong. Could Frances have been implanted with an alien device that tracks her every move; her every thought? Could a Fiend shoot her resulting in a foreign body (bullet) lodged near her Femur artery?
What if the foreign body is Fernando, from Brazil; six-pack abs, smoldering eyes, hips born to Salsa? Run, Frances, Run!
Is Fernando her Fatal Attraction? Will she Fall in love? Will she, of her own Free Will, dive off the Freighter, or a Frigate, to escape his Fixation on her, maybe, she jumps to swim to his ship as it passes in the night?
If so, Bye Felicia!