A/N: set at the end of Season 1 Episode 4 ("Cyberwoman").
As they stood watching Ianto clearing up from the aftermath of Lisa's rampage, Jack knew that there was something more on Gwen's mind than they'd already talked about. He waited for her next question.
"If…" she began slowly, "…if anything like that were to happen again…" she glanced across at him, and away. "Don't let me become a monster," she said finally.
He turned her to face him, his hands on her shoulders. "I won't." She looked up at him, and something in her eyes, pleading with him, told him what she was trying to ask. "If it ever comes to that - and I hope it never will - but if it does, yes, I will pull the trigger myself."
"Promise me."
He pulled her close. "I promise."
She nodded just once, tears spilling down her cheeks as her arms slid round his waist. "Thank you," she whispered.
He cupped her face in one hand, thumb wiping away her tears. Against his better judgment, he bent and kissed her softly on the cheek to seal the promise. He pulled her closer again, comforting and reassuring. "I hope I never have to."