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Well, diamonds are a girl's best friend!

In this clip from Sunday's all-new episode of The Royals, Mandy encourages Princess Eleanor to wear the Koh-I-Noor Diamond, a rare jewel protected in the palace, to her upcoming birthday bash. But unbeknownst to the Princess, it's only part of her shady plan to steal it!

"You have to. You're the birthday girl," she says. "You get what you want. Just have them set it in a necklace and voilà!"

But Eleanor isn't so sure. "It's not even mine though," she responds. "It belongs to the people."

The Royals 206

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Mandy refuses to give up too easily. "Trust me. The people want their Princess to wear it for her birthday," she persuades. "It's just not as pretty behind glass."

"Are you sure it's not too much though?" she asks.

"You deserve to be the center of attention on a normal day," Mandy tells her. "Your birthday? No brainer."

Princess Eleanor is so touched by Mandy's compliment she decides to do a little striptease. Check out the clip above!

Watch a brand-new episode of The Royals this Sunday at 10 p.m., only on E!