Law & Order promised an ambitious crossover, and it did not disappoint. The flagship series and Law & Order: SVU joined forces for a two-part storyline titled "Play with Fire," and it did a great job when it comes to incorporating different characters into a compelling investigation. Here's what you need to know about the crossover, and what went down. SPOILERS, obviously.
The crossover event begins with an ICE raid on a homeless shelter. We watch as two women split up and manage to outrun their potential captors, and agree to reunite at a youth hostel. The only issue is, one of them, "Sofia", never showed. The other women then stumbled upon a burnt body in an oil drum and determined that it was "Sofia." Horrified, she calls Captain Olivia Benson (Mariska Hargitay) to help figure out what happened.
The case was ruled a homicide so it was passed on to Detective Vincent Riley (Reid Scott). It didn't take long, however, for peculiarities to show up in the investigation. Lieutenant Jessica Brady (Maura Tierney) claimed that she had investigated a similar case in which a woman's body was found in an oil drum. Unfortunately, the lead suspect on that case was ruled out for both murders, and Brady realizes that she'd made the wrong call pursuing him years prior. Predictably, the troubled cop beat herself up over the error before forging ahead.
The shocking we alluded to in the title, however, comes into play at this point during the Law & Order crossover. The victim, known as "Sofia" up to this point, is actually Maria Recinos, a police officer that Benson had mentored years prior. Benson actually saved Maria's life when she was a child, and the impact of this interaction led the latter to becoming an officer in the first place.
Benson's former protege gets murdered

Evidently, Maria was trying to build a case against a man who claimed he could provide visas to undocumented migrant women, but was actually kidnapping and trafficking them. The woman who called Benson, revealed to be named Ana, was a friend of Maria's, and knew that Benson was the one officer who could follow through on Maria's findings. The Captain does just that, and manages to find the trafficker, but is forced to rule him out as the murderer.
Benson and Brady attempt to learn more about Maria from Ana by bringing her in for interrogation, but it does not go well. Brady comes on too strong, and while Benson's softer approach proves to be more effective, Ana bolts the first chance she gets. This leads to an argument between Benson and Brady, with the latter voicing concern that the Captain blames her for her bungling the case years prior. Soon after, Ana is found strangled.
The DNA on Ana's body was a match for a man named Miguel Pinto. Our killer, right? Well, Benson and company discover that Pinto couldn't be the killer of all three women, because he was in Staten Island when the first victim was found. The man is being framed, and this discovery segues into the SVU half of the crossover.
This already twisty case takes another wild left turn when Benson and a detective named Gomez (Reinaldo Faberlle) pay a visit to Father Alberto, a priest who was described as an ally to many of the women in the shelter. The priest and Detective Gomez get into a heated confrontation, however, and the former manages to strip the gun away from Gomez before firing it and hitting the detective in the shoulder. Gomez returned fire with a backup pistol, killing Father Alberto in the process.
Elliot Stabler makes a surprise appearance

Benson was befuddled by the whole thing. She felt there was credence to Father Alberto's glowing reputation amongst the women at the shelter, and questioned whether Gomez actually shot himself in an effort to make the struggle between him and the priest seem legitimate. Maybe he was trying to keep Father Alberto quiet? Benson's hunch proved to be right. The Captain discovered that Gomez's DNA was a match for the sample in Maria's rape kit.
Detective Gomez had assaulted and then killed a fellow officer. Benson and Brady were sure of it, and spent the rest of the crossover trying to prove it. Gomez's connects made this trickier than one might expect, however. Not only did the detective called Rita Calhoun, a lawyer known for representing dirty cops, but Benson noticed that a man was trying to talk her son into getting into a car when she went to pick him up from school.
All the pieces eventually fell into place against Gomez, however. Benson and Brady found DNA that matched the detective's on the first victim's remains. Benson also called Elliot Stabler (Christopher Meloni) to look into the man that had approached her son. Stabler took a break from his Organized Crime duties to follow this man (who was revealed to be a cop) and threatened his life if ever talks to Benson's family again.
In the end, Gomez gets locked up, and Benson was relieved that she was able to bring Maria's murderer to justice. The captain goes to Maria's funeral with Brady, and the two cops make peace following their aforementioned argument.