An alien soldier must forsake his own race to protect earth from an armada of slavers.

I am not very good at this but how about concentrating more on the love story??? An alien soldier questions his duties when he is captured not by human soldiers but by the love of a human woman.

As usual, Richiev has a constructive suggestion. And as usual, I get hung up on minor and mega technicalities.

Why would the alien abandon his own species? What is the triggering event?

And as much as Richiev's instinct is right that there ought to be a "B" story -- but inter-species romance? How would that be possible? (The biological bridging gimmick of an avatar has already been used, copyrighted and franchised.)

This is a good beginning of a logline, however; as a rule of thumb, if you say "Must" you should follow it with "Or" then give us the consequences.

Such as:

An alien soldier 'MUST' forsake his own race to protect earth from an armada of slavers 'OR' lose the human woman he loves, forever."

Hope that helps good luck with this! (The logline is getting better)