A former Marines first day at Walmart is turned upside down when eco-terrorist capture the store and take his sister hostage.

First, a spelling pass (yeah, I’m THAT guy, sorry):

“A former [m]arine[‘]s first day at Walmart is turned upside down when eco-terrorist[s] capture the store and take his sister hostage.”

This LL touches all the bases with no excess blather (almost). Protag, antag, backstory, conflict, stakes, setting. Good job. Some suggestions:

– Is the marine doing security at Walmart or a regular-Joe job, like checkout operator? The “gap” (difference between what the protag expects to happen and what actually happens) is wider if he’s doing a mundane job unrelated to his former career.

– “is turned upside down” is unwanted filler. Let’s toss it and flip the focus onto the protag/marine to let him drive the story forward, instead of being acted upon:

“On his first day at Walmarts, a former marine must save his sister when eco-terrorists seize the store and take hostages.”

That hits all your notes and avoids the fluff of “turned upside down”. But honestly, your LL does the job already, Richiev. Go for it. Write what you know (Hooah!) and write what you don’t know: what would I need to do to save my sister if terrorists stormed my workplace right now?)

The character flaw is welcome. We can keep it short by switching to the “PTSD” acronym — it’s common enough, right? And we can do some “2+2” instead of stating outright he was discharged for PTSD. We’re told he’s an “ex” marine, so we can connect the dots with:

“On his first day at Walmart, a ex-marine with PTSD must save his sister when eco-terrorists seize the store and take hostages.

Adding the PTSD gives us a clearer idea how this story might play out, and it directly opposes his outer goal. Nice work.