This is a scene that never got written into The Way of the Universe. You don't have to have read that to read this, but if you have, it takes place somewhere around chapter seven of that.
The Last of Their Kind
"Why did you do it, Helen?" Nikola was compelled to ask one evening over dinner with the woman.
"What?" she asked, not sure to what he was referring.
"Change me back," he replied, looking at her with curious eyes. "You had no reason to feel obligated, in fact, it would seem that my being human, mortal," he cringed at the thought. "Would make your life much easier."
Helen gave him a smile. "That it would." She then took a sip of wine.
"That doesn't answer the question," Nikola pointed out.
Helen sighed as she set her glass down. She took a deep breath. "I cannot bear to yet another person I care about die," she explained, knowing exactly where the conversation would lead.
Nikola grinned. "You care about me," he said smugly.
"Don't get any ideas," Helen smiled, shaking her head. "You are merely the last of my old friends. If I had my choice, I would have saved Watson and left you to die."
"You are so transparent," Nikola grinned. "You love me and you know it."
