Basically, the plot is "Heroine seeks the truth about her father's death." He owned a gold mine on Titan, and died under mysterious circumstances. It's going to be played as a melodrama, with various characters conforming to melodramatic archetypes, except I'll be messing with gender roles a little
So far I have
Romantic Heroine - owns the mine now, wants to know what happened to her father. She is a typical romantic heroine, but a little tougher than most, and has had the sense to hire
The Antiheroine - a hardboiled detective who believes in solving problems with her fists, gun, etc. but doesn't have a lot of experience of working off Earth. She won't be sweeping the heroine off her feet, because this is 1930s SF and neither of them is gay. I'm thinking of a Michelle Hammer type, not quite a psychopath but rather too willing to use violence.
Next we have the Homme Fatale (is this the right term?) - AKA Gold Digger of 2115. He's planning to woo the heiress and (hopefully) parlay their romance into a ticket back to Earth and a luxurious lifestyle.
Meanwhile The Villain is planning his next evil move. This is pretty much the same as above, marry the heiress. Except that this gives him control of the mine, and will leave him free to bump her off at a later date and inherit.
I may put a true hero in there too, someone who will end up getting the girl and saving the day, and possibly a cute kid who will want to play matchmaker.
Any other suggestions?