Forever was a difficult concept to get used to. Henry had first-hand experience of that. Which was why when Claire turned up at his door step that night, he was not surprised.

"I'm sorry," she said. "I just have to know."

Her eyes were red and puffy, and her cheeks were tear stained. She had her hair pulled back in a messy ponytail, and she was wearing her running gear. It looked to Henry that she had gone for a jog, and had just run around and round until she had ended up on his doorstep. That that was where she needed to be.

"Come on in," he said, standing aside.

She walked past him into the shop, navigating around the display of antiques to come stand by Abe's desk. She turned around to look at Henry, her hands covering her mouth.

"Take a seat," Henry invited. "I would invite you upstairs, but Abe is probably sleeping. And downstairs…"

The last time they were downstairs together, Henry had killed himself.

"I'm fine standing," she said, shifting nervously from foot to foot.

"Claire…"

"No," she interrupted. "Henry, I've been thinking – about last night. About your suspicions about my… condition. I can't live with not knowing whether I can or cannot actually die."

Henry sighed.

"What brought this on, Claire?" he asked.

"When I woke this morning, I told myself that everything was fine. That everything was normal, and I went back to work," she explained. "I tried to pretend that what I could be didn't matter. But being surrounded by my life, and knowing that someday I might still be here but everyone I know and love might not be…" She shook her head. "The risk of having something that I could lose seemed a far more painful reality than never having anything to love at all. How am I meant to live my life if I don't know what I have to lose?"

"Everybody goes through their lives with that possibility, Claire," Henry said. "Whether you are immortal or not. Mortality does not insulate you from the risk of loss."

"But immortality confirms it," she said.

"Claire, I have walked this path for over two hundred years now," Henry said. "Immortality is no reason to close yourself off from the world. A life lived forever alone is no life at all."

"But I still have to know," she said. "Even if knowing doesn't change the eventual outcome."

"The only way for us to confirm if you are truly immortal is for you to die again…"

"I know," she said. "That's why I'm here. Henry, I need you to kill me."

Henry froze.

Henry had been asked that request before – by a young man, kneeling in his laboratory, a katana at his knees.

I want you to kill me, with this.

When he had asked, Henry had been convinced that this man was Adam. Convinced that he was immortal, as he was. And he had stabbed him through the heart with a letter opener, when he had thought Abraham's life was in danger. When he thought that the only consequence he would have would be him disappearing to be reborn elsewhere.

He had been so sure. But he had been wrong. He had watched the light fade from Clarke Walker's eyes, never to return. He had felt his body go limp and slump to the ground, where it would remain.

He had been so sure.

"No," Henry said.

He could not take that risk with Claire Gage's life. Even though he was sure of her condition. He had been sure in the past.

"Henry, please," she said.

"No," he repeated.

"If you don't," she warned. "I'll do it myself…"

Claire placed a hand on Abe's desk and picked up a letter opener – a letter opener, of all things! – and brought the tip to her throat.

"Claire – don't," Henry warned, stretching a hand out towards her.

"Henry, I have to know," she said.

"Not like this," he begged. "Please. Claire, immortal or not, death is still real. There is still pain. And the memory of each and every one will remain with you for the rest of your life."

"I need to," she said.

Henry could see her hand shaking. He watched as grim determination set on her face. She tilted her face to the side.

"I'm sorry…"

"Wait!" he said, throwing his hands up in the air. "I'll do it. Please, Claire, just put the letter opener down."

Claire wavered.

"You're bluffing," she said.

"No. No I'm not," he said. "But let's be slightly more clinical about it. You puncture a carotid and die, and I am wrong…"

He shook his head.

"I will be unable to help you. But – just maybe…"

He rubbed a hand through his hair.

"I have a defibrillator downstairs," he explained. "Don't ask why. I just needed it for an experiment once. If I shock your heart, I can stop it. You will be clinically dead."

He looked at her, hoping she would understand.

"If it doesn't work, you plan to shock me back," she said.

"Yes," he said, relieved. "Now, I do not know if this will work. I do not know if it's only permanent death that causes us to disappear, but I am willing to try. It does not completely exclude the possibility of immortality, but it would prove it."

Claire nodded.

"Okay, Henry. Let's do it your way."

Claire lay on the barouche in Henry's lab, ECG dots stuck to her chest. The defibrillation pads had already been applied. Abraham stood in the corner, leaning against Henry's desk. He had not been pleased when his father had woken him up to get him to assist him in his, quite frankly, insane plan. But Henry needed him. If this worked, then he needed someone to go find Claire. And if it didn't… Well, he might need someone to call an ambulance while he attempted to resuscitate her.

"Are you ready?" Henry asked, fiddling with one of the nobs on the defibrillator.

Claire nodded.

"Ready as I'll ever be," she said.

"Are you sure?" Henry asked. "I mean, I may be ninety-nine percent sure this will work, but there is still a chance…"

"Don't worry, Henry," she said. "I won't blame you if it doesn't. I'm ready."

Henry nodded, and took a deep breath in to gather his courage.

"I'll see you soon, Claire," he said.

He pressed the button.

Henry watched as Claire convulsed, then went still. The ECG lines on the screen were a haphazard mess. R on T phenomena, they call it. Where a QRS complex is superimposed on the repolarisation phase of the conduction of a heartbeat. It induces a ventricular arrhythmia that stops the heart. It had done exactly what Henry thought it should.

But Claire did not disappear.

"Henry – come on," Abraham said, anxiously coming over to where his dad stood. "It didn't work. Now bring her back."

"Not yet," he said.

"Dad!" he said.

"Not yet, Abraham!" Henry yelled.

"You're killing her," he said.

"That was the point of the exercise," Henry said. "But we have to give this a chance. Claire is risking all this so we can know."

"So you can know," Abe said. "Dad, this is insane. Why are we risking this girl's life on a theory?"

"It's not a theory."

"Then why is she still here?"

Abraham was right. Claire was still lying on the table, sixty seconds after Henry had stopped her heart. Her ECG had changed from ventricular fibrillation to asystole. She truly was dead, and she was still here.

And then she wasn't.

"Holy shit," Abe said. "You were right."

Henry did not even think to tell Abe off for his language, like he normally would. He just stared at the empty bed, where Claire had one laid, and said:

"I know."