Blue Bloods Season 9 Episode 21 revolves around a murder Danny and Maria are investigating where their main suspects are a set of twins. The murder is nearly impossible to solve because both twins have the same DNA and offer the same alibi, but Danny knows that one of them killed the victim. But can he prove it?

Danny and Maria are at a loss until Danny gets an idea. Could one of the suspect's ex-girlfriends be the key to solving this murder?

Meanwhile, Henry faces a dilemma of his own when the necklace he bought for Nicky's graduation is stolen off of his porch. When Sean suggests Henry introduce himself to his neighbors and see if anyone saw anything, Henry hits pay dirt right away. The thief is the granddaughter of his neighbor Donna, and the girl is an addict. Grandma has had enough of Alexis' shenanigans, but can she convince Henry to press charges?

Finally, Jamie gets caught in the middle when Eddie's father asks to be furloughed from prison for their wedding. Eddie doesn't trust her father, but will she let him attend anyway?

To find out what happens, watch Blue Bloods Season 9 Episode 21 online.