A Space Force hires a mercenary to get other recruits to take down their mentor who betrayed them.

A mercenary hires other space cadets to save his girlfriend from aliens invading Earth.

Sorry, I was a bit vague... I am saying start with the lead character. The one(s) recruited to do the job.

"When he's hired to kill his mentor..."

They are hired to kill their mentor.

These guys hire another guy, to recruit some other guys to do a job...

...Take the long way home