Things aren't going so well for Peter Stone (Philip Winchester) in his final appearance as a series regular on Law & Order: Special Victims Unit. Thursday's season finale finds the ADA in a tough spot after federal agents dropped the charges against Rob Miller (Titus Welliver), the prime suspect in his latest case.

In this exclusive sneak peek at the episode, titled "End Game," Stone breaks the unfortunate news to Benson (Mariska Hargitay) and Fin (Ice-T) who, like Stone, are horrified that their suspected rapist will remain a free man for the time being. Leave it to Fin to perfectly capture the sentiments of the room in a single-syllabic word.

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"Damn!" he exclaimed.

Equally dismayed, Benson broke down the facts of the case to Stone and made it clear that they had solid evidence against the suspected perpetrator. "He blackmailed a judge. He threatened witnesses. He threatened Nikki Staines. We have him on tape saying, 'No Nikki, no case,'" she explained.

However, a federal judge deemed their intel insufficient which blew a huge hole in their operation. It didn't help that their key witness changed her story and claimed the act was consensual, garnering exasperated looks from Benson and Fin.

"End Game" will see the SVU squad investigating the death of a teenager who was found in the Hudson River. Believing that Miller is their guy, the gang face a staggering task ahead to prove his guilt with only circumstantial evidence and very little time to close the case. The episode marks Stone's final appearance as a series regular after it was revealed that he will not return for the show's historic 21st season.

Law & Order: SVU's Season 20 finale airs Thursday at 10/9c on NBC.

Philip Winchester, <em>Law & Order: Special Victims Unit</em>Philip Winchester, Law & Order: Special Victims Unit