Belle Gugu Mbatha-Raw, Tom Wilkinson, Sam Reid. Directed by Amma Asante. Rated PG. Opens Friday.

As a period piece, Belle follows two pretty familiar formulas: It’s a love triangle centered around a woman of means and stature who must choose between a proper gentleman and a passionate rogue; and it’s also a historical drama about a landmark legal case that was a catalyst for political change. By combining the two, director Amma Asante and writer Misan Sagay are able to shake up some of the stale trappings of the genre, at least until the speechifying gets a little out of hand at the climax.

It also helps that the title character isn’t the typical period-piece heroine: Dido Elizabeth Belle (Gugu Mbatha-Raw), born in 1761, is the daughter of a British naval officer and an African slave, raised by her great-uncle, Lord Mansfield (Tom Wilkinson), who is also the Lord Chief Justice of England. Dido’s upbringing is as proper as any English lady’s, but her racial background prevents her from enjoying the full advantages of her family name, and keeps her at a constant distance from her full-blooded English relatives. Mbatha-Raw gives a strong performance that balances romantic and political passion, and Asante gives both sides of the story equal weight. Belle finds its strongest moments in making the personal political (and vice versa).