The Three Musketeers (1948), directed by George Sidney.
The first half -- the adventure of the queen's diamonds -- is light and very silly, taking full advantage of Gene Kelly's athletic ability and comic mugging. The second half -- the exposure of Milady de Winter -- is darker. We might expect that she and Constance would be spared in this version, but it is not to be.
The two-part structure was used again by George MacDonald Fraser in his fine screenplay for The Three Musketeers (1973) and The Four Musketeers (1974). He praises this version in The Hollywood History of the World and I suspect he had it in mind when he wrote his own treatment. It's impressive how much of the original story gets covered in the films.
As adventure entertainment it is only so-so: the light costuming and California country locations take us out of the fantasy, but we have exciting sword fights and impressive acrobatics.
A fine cast:
Gene Kelly: brash, love-struck D'Artagnan -- a real historical character!
Lana Turner: wicked Milady, keeping a dark secret
Van Heflin: anguished Athos
Vincent Price: sinister, crafty Richelieu, apparently not a Cardinal in this version
Frank Morgan: ridiculous but pitiable King Louis; we hate to see anyone under Richelieu's thumb
Angela Lansbury (age 23): the adulteress Queen, although we never seem to mind
June Allyson: D'Artagnan's lady love; they marry in this version and we have a Romeo & Juliet morning-after bedroom scene
Keenan Wynn: long suffering servant Planchet
This must have been spectacular Technicolor when it was new, but the DVD quality is only fair. Will we ever see a restoration?