Spoiler alert! 'Law & Order: SVU' 500th episode dives into Benson's past, as former stars return

Spoiler alert: This story contains details from the Oct. 21 episode of "Law & Order: SVU."

Olivia Benson has reported for duty 500 times. This time she has to think a lot more about herself.

NBC's "Law & Order: SVU," TV's longest-running drama, aired its 500th episode Thursday, a huge milestone for the crime series in its 23rd season. The show marked the occasion with an episode focused on star Mariska Hargitay's Olivia Benson, forcing her to reckon with a troubling part of her past.

"The Five Hundredth Episode" finds Benson and the rest of the SVU team looking into a cold case after former detective Nick Amaro (Dany Pino) brings it to their attention. He's now working for a company that specializes in analyzing cold-case DNA with new technologies.

Nick was contacted by a journalist and podcaster investigating the 1996 case of a teen girl who was sexually assaulted and strangled. Her boyfriend confessed to the crime and was convicted, but he later recanted, and Nick and his partner believe he's sincere.

When Nick brings his partner in to meet the team, Benson is stunned speechless. She calls the man, Burton Lowe (Aidan Quinn), an old friend, but through a series of flashbacks the audience discovers that he was more than that: He was her boyfriend as a teenager. She was 16, he was 21. He asked her to marry him, she said yes, but eventually she ended things at her mother's urging. Seeing him so many years later is unnerving, forcing her to relive painful memories with her mother and to reflect on her first relationship. In working on the case, she and Burton reconnect romantically.

Benson remembers the case: It was one of Cragen's (Dann Florek) back in 1996, and she agrees to let Nick, Burton and the squad look into it. Cragen offers his insights, appearing via video chat to bring a touch of nostalgia to the 500th episode.

Thanks to a small DNA sample that Dr. Melinda Warner (Tamara Tunie, also making a return appearance) finds on the old evidence, Nick's company proves that the boyfriend was wrongfully convicted. The real killer – a teacher from the girl's school – is soon discovered, and he confesses out of long-held guilt.

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But just when you think the mystery is solved, a woman confronts Benson and Burton outside the court where the new killer has just confessed, asking the captain of SVU why she would work with a predator.

The woman comes down to the station and tells her story: A decade ago, she worked with Burton and that he groomed her and raped her one night when she was too drunk to consent. Her story of grooming, from fancy dinners to sending her music like "The Girl from Ipanema," stirs Benson's memory. In her house she searches through an old box of cassette tapes and finds one Burton made for her when she was a teen with the same song on it.

After Burton's accuser posts her experience on social media, other women come forward. They describe a predatory pattern in Burton's behavior, sexual harassment and manipulation of women. None of their allegations rise to the level of a chargeable criminal offense, so SVU is forced to simply move on. But Benson can't: She goes back to Burton's hotel room, where only a few nights before she was a happy guest, and confronts him about their relationship. She was too young to consent to have sex with him as a teenager, she tells him, and their relationship was inappropriate. She wants Burton to acknowledge his wrongdoing, but he refuses.

The episode ends with Benson walking away from Burton's hotel room but stopping to throw the old mixtape he made her into a pond.

The return of Nick, Melinda and Cragen was a treat for viewers who have stuck by the show since it made its debut in 1999, but there wasn't much in the way of fan service or nostalgia in the episode, besides Burton's hotel room number (500). It was a dark, serious outing for an already dark, serious show, reminding viewers that even 500 episodes in, the writers and actors aren't slowing down or getting lazy in creating shocking and dramatic storytelling.

"SVU" will have another cousin back on TV when NBC brings back the original "Law & Order" next year. But even if that revival works, it probably won't have someone like Olivia Benson as a grounding character.

Court is back in session: NBC will bring back original 'Law & Order' for 21st season

This article originally appeared on USA TODAY: 'Law & Order: SVU' 500th episode dives into Benson's past