Trixie is already an established bounty hunter when she decides to spread her "wings" as it were, and go out to Las Pegasus to take a job for an industrial owner who's having some trouble that could be solved by somepony with Trixie's skills.
She'll face a surprise or two along the way, and through this wild night in Las Pegasus, she'll get more than she bargained for, but she'll certainly take another step towards becoming Equestria's greatest bounty hunter, that's for sure.