When "Law & Order: SVU" returns to NBC for its 18th season, Vice President Joe Biden will be one of the series' notable guest stars -- marking his second time on a NBC scripted series. (The first time opposite Amy Poehler's Leslie Knope on "Parks and Recreation.")

"A dream come true," Mariska Hargitay, who plays Lt. Olivia Benson, tells ET about Biden's upcoming appearance during a visit to the set in September. "Joe Biden is one of the greatest people on this planet."

Appearing as himself, Biden opens the second episode -- airing on Sept. 28 -- titled "Making a Rapist" about a convicted rapist (former child star Henry Thomas of "E.T. the Extra-Terrestrial" fame) exonerated due to newly tested DNA evidence after 16 years in prison. A day later, the man comes under investigation when he's suspected of a gruesome crime.

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The episode reunites Hargitay and Biden, who both spoke at the United State of Women Summit at the White House in June to address the very issue at the heart of episode two. Biden was also honored by Hargitay's Joyful Heart Foundation last May for his commitment to ending domestic violence and sexual assault.

"He and I do a lot of work together, so it's been something that's been talked about," Hargitay says about trying to get the vice president to appear on the show, before bringing the idea to Rick Eid, who took over as showrunner this season. "[Rick] wrote such a beautiful scene."

While Hargitay has a close relationship with Biden, Peter Scanavino (Det. Sonny Carisi) says it was a humanizing experience seeing a world leader asked to do multiple takes. "It's like, 'Wow, you're a human being just like the rest of us,'" he says.

Though, Biden was only available for so many takes before he had to get back to his duties as vice president. "He was like, 'OK, if I have to do this again, Ice, you're going to have to get on the phone with some foreign dignitaries,'" Ice-T (Det. Odafin Tutuola) recalls.

"He really did have a phone call scheduled," Kelli Giddish (Det. Amanda Rollins) added.

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Biden is not the only notable guest star this season. Anthony Edwards is set to appear in an upcoming episode, marking an an onscreen reunion for him and Hargitay, who played lovers on season four of "ER."

While the cast wouldn't divulge in any details about his character ("You're not getting that out of me," Hargitay teased), the actress did open up what it means to come full circle with her time on the long-running NBC medical drama. "I learned so much about who I wanted to be, what kind of actor I wanted to be, how I wanted to conduct myself on a set. Truly, from that show," Hargitay says. "Anthony was so protective of me. Now, we're family friends and he's been involved with my foundation."

"I've been begging him, so this is a gift."

Between Biden and Edwards, Hargitay ended up with two gifts this season, which returns to NBC on Wednesday, Sept. 21.