The Constant Gardener shows us what happens when a film's moral argument outweighs its story. The film has a serious thesis it wants to express concerning the plight of Africans and the responsibility of pharmaceutical companies that supply them with drugs. There's nothing wrong with starting with a theme and creating a story from that. But it had better be a good story.
In The Constant Gardener the writer chooses the thriller and love story on which to hang the thematic line. A diplomat's wife is killed and he sets out to find out who did it and why. This brings him into considerable danger of being killed himself. He learns that his wife had discovered truly horrible crimes committed by drug companies in Africa.
To make this work, the writer has two big requirements. First he has to show that this was a great love between husband and wife, because the husband must risk his own death to finish the job his wife started. Second, the writer must come up with a detective plot that is full of reveals and surprises, or else the audience is going to see early on that this story is just an excuse to attack international drug companies in Africa.
Unfortunately the writer fails in both of these requirements. The husband and wife meet at a lecture, go to bed together in the next scene, and then head off to Africa as husband and wife. The wife doesn't trust her husband enough to tell him about the secret investigation she is pursuing. And there is little evidence that their marriage is anything but a convenient connection between two good friends.
It's one of the great rules of storytelling that you can't montage love. An audience can't intellectually know that two people love each other. They have to feel it, and that takes screen time. Without the foundation of a strong love between the two characters, the husband's quest to uncover the injustice, in the face of almost certain death, is emotionally unbelievable. And the quest driving three quarters of the movie just falls apart.
The writer also fails to come up with a detective plot to justify the length of the story. Detective stories work by withholding information from the audience. If that information, in the form of reveals, is not surprising or shocking, the story feels like a giant stall. The wheels of the mechanism show and the audience gets impatient and bored. If the theme is top-heavy to boot, the lack of storytelling ability is fatal.
