Lanie, Esposito and Ryan are furious that they weren't invited to the wedding. Plus, the happy couple's tropical island honeymoon will have to wait since Kate used all of her vacation time trying to find Rick when he disappeared. 

Whitney is poisoned while at the Diamondback Old West ranch in Arizona. She ended up dead by the time she travelled back to New York.  When the local sheriff won't help, Kate and Rick go undercover as a honeymooning couple at the ranch.

Whitney stole a ranch shed key from a ranch hand to steal dynamite. She also wandered off from a horseback riding tour to find something on the Yavapai Reservation. She asked about a Yavapai word. It could be river or it could mean stream. 

Whitney had a 22 year old letter from Clyde (the father she never met) saying that he and his friend Dutch were close to uncovering the key to the secret of the Peacock Boys. Rick and Kate figure out that The Peacock Boys stole 50 gold bars and then stashed it. The story says they stashed it near a river but the translation is wrong. It was actually a stream and it disappeared when it was damned off in the 1920s. 

Whitney found where the gold was hidden but the gold was gone and Clyde's remains were in its place. Dutch and Clyde had a third partner who didn't want to turn the gold in for the finder's fee. James Grady, proprietor of the ranch fought with Clyde and Clyde died when he hit his head on a rock. Then Grady split the 50 bars of gold with Dutch. When Grady realized that Whitney had figured out his secret, he poisoned her.

When Castle figures it out, he and Grady end up drawing pistols on one another…almost. Beckett shoots the gun out of Grady's hand. 

Capt. Gates convinces Esposito and Ryan to stop whining about not being invited to the wedding and do something nice for Castle and Beckett. They each donate two vacation days to Beckett so that she and Castle can have a few days to honeymoon. Kate chooses to stay at the ranch, as long as they take their tropical island getaway another time. Castle happily agrees.