'Castle Rock' Episode 4 Recap: Spoilers For The Box
Many things can be a box: a prison, a home, a mind. At the beginning Castle Rock's episode 2, "Habeas Corpus," you'll remember that through the security cameras, Officer Dennis Zalewski (Noel Fisher) saw the Kid (Bill Skarsgard) escape from his cell and embark on a murderous rampage throughout Shawshank Prison, killing every guard he came across. After an accidental misfire at his coworkers, all of them still alive, Zalewski realized that it was a false alarm. The Kid was still in his cell. He had hallucinated everything.

In that same episode, the Kid's new cellmate, an enormous Neo-Nazi, backs him into a corner. The Kid whispers, "You don't want to touch me." The next morning, the Nazi is dead; his whole body overcome by tumors.
"The Box" opens as Reeves (Josh Cooke) visits the Kid in his now-empty cell and threatens him for information. In a terrifying moment, the Kid backs Reeves out of his cell, reciting Revelations: "He has a name written on Him, which no one knows except Himself. He is clothed with a robe dipped in blood. And His name is called the Word of God."
A terrified Reeves calls for help but the Kid doesn't touch him. He closes the door to his own cell. Through this act, he lets all of Shawshank (as well as the audience) know that he has the power to leave, but he chooses to stay.
As the mystery surrounding the Kid continues to grow, Henry discusses his flashbacks with Molly: a toy truck, a shack with a dirt floor. The title of the episode begins to make more sense when a desperate Zalewski confides in Henry about the horrors he's seen in the prison: "I'm a prisoner in there, too."

Shawshank is a violent place: There are beatings in the yard, the cafeteria, and the cellblock. Witnesses stand by and do nothing. Zalewski promises the Kid he will help get him out. He reaches through the bars for a fist bump as a sign of comradery. It's the first time someone besides the Nazi has touched him. When their hands meet, Zalewski immediately begins to exhibit odd behavior.
Meanwhile, it is finally revealed that Henry Deaver was, in fact, kidnapped. His memory loss and missing time are a result of the traumatic event at a young age. The "shed" is actually a large wooden box on the Desjardins' property. When Henry breaks the lock open, something else breaks open in his memory.
After he is confronted with one of his possible kidnappers, Henry decides to take the plea deal from Shawshank for the freedom of the Kid so he can leave Castle Rock as soon as possible. He calls Zalewski to tell him that they will not be testifying in court. The Kid will be released and the prison will continue the abuse of its prisoners. In a daze, Zalewski leaves his post once again with his gun drawn. The prison guards fall one by one as Zalewski shoots his way through the prison. In episode 2, the violence he saw on the security cameras was not a hallucination after all, but a premonition of what was to come. With the blood of his coworkers spattered across his face, Zalewski tells Henry: "I wanna testify."

These are his last words. "The Box" ends with a single shotgun blast from another prison guard. Zalewski, Henry's only witness to the crimes of Shawshank, bleeds to death in the warden's office, never to leave the prison again.