An ice cream man, Anton is found shot to death in his ice cream truck. An unidentified child was hiding in the truck and left behind a permission slip with from Mrs. Leslie Ruiz's 2nd grade class. When no child from the class will admit to being in the truck, Beckett decides to send Castle into the classroom undercover to gain the student's trust.
Castle's experiment doesn't go as planned. He bonds with the kids, playing Army men and having fairy tea parties but the mystery witness doesn't come forward and the Principal asks him to leave when he tells little Emily to find her inner strength when dealing with Jason, a bully. Emily then punches Jason in the nose.
Beckett's investigation shows that Anton and a retired police officer named Clarke Jaffe were using a mail box place as a drop site. Jaffe was killed the same days as Anton. Before Anton was killed a Russian man called to ask who owned the mailbox. When Beckett and team track Dmitri Kalenkov down, they find he's been killed and was killed before Anton and Jaffe.
Somone breaks into Anton's parent's home and search it. His father later finds a stash of blank passports.
As Castle is leaving the classroom, a child slips a picture if an ice cream truck into his pocket. Mrs. Ruiz recognizes the picture as one of Jason's, the bully of the class. Jason says his adult sister was in the ice cream truck but told him not to tell. Her name is Natalie Barnes and she was Anton's teacher and girlfriend.
Anton, Jaffe, and Natalie were making fake passports for Russian immigrants whose paperwork had been taken by unscrupulous employers who forced them to work in sweatshops for little to no money. Anton wanted to make the passports to allow them to get back home but he recognized one of the photos as Polkovnic, a war criminal who took out entire villages back in his home country of Georgia.
Polkovnic is wanted for international war crimes but there are no pictures of him. When he found out that Anton knew who he was, he killed him and Jaffe looking for the photo. Polkovnic hears Beckett tell Castle that little Jason unwittingly has the photo in a camera case he took from his sister's as he hides in Natalie's apartment.
Back at the school, Castle and Beckett find the photo just as Polkovnic arrives and attacks them both. As Polkovnic is about to get away, Castle throws a jar of marbles on to the floor, sending Polkovnic flailing to the ground where Beckett arrests him.
Later, Castle talks to Alexis about her over-protective behavior towards him since he's been back. He assures her that he knows how she feels. He felt the same way after she was kidnapped and taken to Paris but being an adult means realizing that you can't always protect the ones you love. Then the two head out for pizza.