Benson and Stabler are confused as to why they're called to the scene of a fire where two little girls were found dead. Warner tells them it was their new ADA who called in the detectives. Stabler is shocked to learn the new ADA is his old partner Jo Marlowe. Benson's surprised that after all this time, Jo is in fact a tall, attractive woman.
Stabler and Benson rush to Frank's hospital room to interview him. Frank's still distraught that he couldn't save his two little girls from the fire in his apartment. Back at SVU, Fin tells the detectives that Frank had custody of both his daughters because his ex-wife failed the psych exam at the custody hearings.
The detectives hear about 23-year-old Michael Parisi, who had been harassing 10-year-old Kedzie just days before the fire that killed her and her sister. When Benson confronts Michael, he tries to run, but gets caught. Benson finds a mini blowtorch and lighter fluid in Michael's pocket. Stabler discovers Michael has a rap sheet for harassment.
Stabler interrogates Michael as Marlowe and Benson look on. Stabler tries to break Michael by lighting a fire in front of him in the interrogation room. Stabler gets Michael to admit he groped Kedzie once and confess to arson as well.
Marlowe worries her case has been destroyed by Stabler's risky interrogation - using Michael's obvious arousal from the fire to coax a false confession. She and Benson head back to the Frank's apartment, where the fire marshal tells them Frank obviously lit the fire that killed his two daughters. Benson arrests Frank, who smacks Marlowe in the face before being handcuffed.
After discovering insurance fraud was the motive for the fire, Benson questions Frank's oldest daughter Emily about what she knows. Emily begins to talk about her father's money problems. Afterwards, Stabler interrogates Frank, who continues to maintain his innocence and asks for a lawyer.
At his arraignment, Frank is ordered to Rikers until trial. While enjoying takeout in her office with Stabler, Marlowe receives a call from the prison. Frank's ready to meet with her. At the prison, Frank tells Marlowe that when he discovered his lawyer thought he was guilty, he fired him. Now he just wants to meet with Marlowe one-on-one.
Maintaining his innocence, Frank start confessing all the terrible things he did in the past, including tax evasion and cheating during his marriage. He knows he's far from being the perfect dad, but by no means is he a murderer or arsonist.
Frank takes out a knife he smuggled into prison and carves up his arm. The police wrangle the knife away from Frank, leaving Marlowe visibly shaken. Later, she tells Benson and Stabler she believes Frank's declaration of innocence. Stabler thinks Marlowe's crazy, but Marlowe sets out to prove she's right.
Marlowe travels to Iggy Drexel's farm in Ellensville so he can look at her investigation file. Marlowe says Iggy has the reputation of being the best private investigator when it comes to fire. Iggy has a problem with the fire marshal's findings, simply because he doesn't trust fire marshals to begin with. Iggy says the entire investigation was obviously half-assed.
Iggy demonstrates to Marlowe how the fire really could have ignited, starting with the apartment's old windows. Accompanied by Iggy and Stabler, Marlowe brings Frank to the scene of the fire and tells him to recreate exactly what happened the night his daughters died.
Iggy describes what he thinks must have happened when Frank tried to save his daughters, as the fire was spreading rapidly inside their bedroom. When the fire traveled throughout the rest of the house, it left a smoke trail, making it look like arson. Iggy's conclusions make sense, but don't completely eliminate the fire marshal's theory, either.
Stabler, Benson, and Marlow return to SVU to find an angry DA McCoy waiting. McCoy tells Marlowe that since he's her boss, she needs to pick up the phone when he calls. He's not happy she's trying to disprove the State's case against Frank. McCoy tells Marlowe she's being snowed by Frank, and orders her to bring the case to trial.
McCoy asks Stabler if he believes the fire was accidental, per Marlowe's findings. Stabler tells Marlowe he has no reason to doubt his former partner. McCoy asks the same question of Benson, who doesn't know how to answer. Again, McCoy tells Marlowe go to trial, but Marlowe has a better idea. First, she's going to recreate her own fire with Iggy to see if it yields the same results. Afterwards, she heads to Frank's trial with new information.
In court, Marlowe grills the marshal who declared the fire intentional, showing him the footage she shot with Iggy. The fire marshal testifies that the fires are identical - both had to be intentionally started in the same way. That's when Marlowe tells the jury about the fire she and Iggy recreated... using the same heater that was in the girls' bedroom, and the same old wood and glass their house was made of. The fire marshal's theory is wrong, and Frank is innocent.