House had been observing that Thirteen behavior after learning that she had Huntington's had entered a new phase suddenly. She had been taking risks to indulge in a lifetime's worth of thrills in a few years. Now suddenly she seemed to be making long term plans. This intrigued House. Also, he had noticed that just before this change in behavior someone had been admitted to the hospital under a false name reserved for celebrities, an action only Cuddy could approve. This patient was only there for about eight hours and those hors were the night before Thirteen had been absent with no explanation. Cuddy likewise had gone missing. Both events were out of character.
As he was pondering his scheme to find out if there was a connection, a custodian he'd befriended mentioned that a hospital bed had needed to be replaced because it had been damaged. The custodian had mentioned that from his time in the military he'd noticed that there was the smell of gunpowder which led him to check the sheets being trashed and he'd noticed the stippling from a gun fired at close range around a bullet hole. The custodian save the sheet for House and it contained considerable a blood. A check the room number revealed that this was the room of the mystery patient. The shooting could explain why the patient was there for such a short time. Digging deeper, House found that the doctor for the mystery patient was Adam Pierson and found out that there was no doctor with that name.
When House went to Cuddy, she first denied that the false name had been used, but after being pressed she simply told House to let it go and devote his attention to his patient. House met Wilson for lunch in the cafeteria and then returned to his office.
As House walks into his office he finds a man casually sitting at his desk drinking a beer and bouncing House's ball off the floor and wall catching it in his free hand. The man is wearing a dark trench coat. "Don't get up." House says as he walks in. "Thanks" the man says continuing to play catch with the wall. "You are Gregory House I'm thinking." He adds.
"And you are….?" House asks. "I'm just a guy that your friend Cuddy brought in to solve a case you couldn't." The man answers and House just looks at him in silence. "Actually, you could if you knew what Cuddy and I knew. The solution was obvious, if one had the will to implement it."
"You would be Doctor Adam Pierson I assume." House asks. "I would. Want a beer?" the man asks. House shakes his head but asks. "Who was the patient?" Adam smiled and responds "Someone you know, that's why Cuddy used the false name."
"I've checked the registers and you're not a doctor." House says trying to get a rise from the casual stranger. Adam shakes his head "I was once, University of Heidelberg class of 1453." Memories of a conversation with Cuddy flood back into House's mind. "Want me to join your team?" Adam adds with a smile.
"House, Cuddy hoped you would not have noticed…" Adam says as he walks to the door, locks it, and closes the blinds. "But she knew if you did you would not stop until you had some answers…. And it is important that you do stop. People like Cuddy and I live in secret. We also stop aging and become immune to disease when we suffer a violent death. Before that happens, we live normal lives ignorant of what we are. Afterwards, we can sense others like ourselves. But we must learn everything else from others of our kind."
"Some of this I heard from Cuddy awhile back when a guy came back to life in the morgue…" House observes. "Yup, Duncan. He wasn't the first and he won't be the last." House nods. "How many…?" he asks. "How many like me?" House nods. "Not many, I can go decades without meeting another.
"It was Thirteen. Cuddy was unwilling to give her a violent death herself, but she knew it was the only way. I don't know how she convinced her, but she distracted her so Thirteen wouldn't see it coming. Single shot straight through the heart. It was over quickly. We took her to the morgue and cleaned her up and waited for her recovery. Cuddy will teach her how to survive. But it is important that nobody knows, otherwise Thirteen will need to start a new life somewhere else." House nods slowly.
"Oh and one other thing… Cuddy wanted me to tell you… It's April 1." Adam tosses House the ball and strolls out.
House is unable to focus for the rest of the day but he observes Thirteen closely.
As he gets on his bike to leave at the end of the day, Cuddy drives by with Thirteen in her car and they both smile and give him a small wave.