Frances had presented herself as a loving mother to David, but as revealed during the progress of the episode, she is actually a hateful bigot. She filed for divorce from Josh after 20 years of marriage right after she learned that Josh (who looked like and could pass for a White man) was actually Half Black; learning this after meeting Josh's sister. Frances even fought against having custody of David, mainly due to him being part Black. She gained custody, but only if Josh paid three times more for alimony.
Josh later remarried to Karen and they recently had a baby girl Named Maggie.who was born with Black physical features. Having learned this, and also having learned that Karen planned to keep the baby over giving her up for adoption, the evil Frances confronted and later killed Karen by shoving her over the balcony, but not before referring to the baby as a threat to her status--while referring to the baby as (according to Jerry O'Brian) "a nappy-haired little monkey." After the murder, Frances took the baby and gave him to the O'Briens, with orders to take her out of town.
Due to the baby's skin color, Detectives Briscoe and Curtis were led to believe that Karen had an adulterous affair with a Black man, but they learned that Josh was actually Black and the biological father. He was suspected of the murder due to the fact that he claimed that he was White in job applications, but after his ex-wife's DNA was found at the scene, Frances ended up arrested and charged with Karen's murder. During her testimony, Frances continued playing the loving mother, stating that she confronted Karen to protect David from racism that he would deal with at his school; a school that is all-White and that she enrolled him in.
However, Frances' true colors were revealed when Jack McCoy and Jamie Ross uncovered the divorce settlement, which showed that she didn't want custody of David. Josh refused to testify to this at the next trial, but McCoy and Ross told Frances that he would do so during a meeting with her and her lawyer. As a result, Frances accepted a plea deal; second-degree manslaughter carrying a sentence of as little as 8 1/3 years. Following her acceptance, Frances snarled at McCoy, telling him that he had no idea what she went through, only for McCoy to respond that it was all in her mind.