Nothing But the Truth (1995)

In a small town, a polygraph examiner learns that a doctor she's just met is the prime suspect in a sex murder. She administers a polygraph test which clears him, and they proceed to have an affair. But then a witness places him at the murder scene, and he's found to have concealed being a suspect in a previous murder elsewhere. The evidence mounts, the doctor is arrested, and the woman tries to keep believing in him and find the evidence that will clear him.