After surveilling Jason Marks all night, Beckett is shocked to find out he was murdered. When Vulcan Simmons looks to be involved, Gates takes Beckett off the case.
Later, Kate is set up for Simmons murder and she and Castle go on the run. The find out that the mysterious Mr. Smith faked his own death and has been searching for a cassette tape that has Bracken confessing to murder from years earlier but he can't find it.
Kate escapes Bracken's attempts to kill her and in a dream about meeting Capt. Montgomery for the first time, she realizes that the cassette was his and he gave it to her mother to hide. Castle and Beckett try to decipher her mother's code from her journals but are arrested along with Esposito and Ryan who were helping them.
At the precinct, Kate realizes that the code referenced the elephant family that had been her mom's but now resides on her desk. Kate breaks it open and finds the cassette. She later arrests Senator Bracken for Johanna Beckett's murder. Kate tells Castle she could have never done it without him.