Keri kicked a few unopened bills out of the way as she, Sherlock, John and Megan entered the apartment over-looking Mermaid Quay in Cardiff. Megan flicked on the light switch as Sherlock started examining everything he came across.

John looked at him out of the corner of his eye, swallowing. Keri leaned towards the doctor, smiling.

'So.'

John's eyes snapped to meet hers. '...Hm?'

She slid closer towards him, arms folded. 'You alright?'

He reddened slightly. 'Yup.'

'It's really cool, all this detective stuff.'

'Mhm.'

'It's kinda...' She dipped her head towards his. '...Exciting.'

'Yeah.' John said, paying more attention to the consulting detective with his back to him. Keri smirked and moved away.

'I am so going to win this.'

John gave her a sideways glance. '...Huh?'

'Exactly.'

Sherlock opened the door to Vanessa's room slowly, fishing out his pocket magnifying glass. Megan followed him, trying blindly to look for whatever the hell he was looking for. She gave up when he dropped to the floor, examining the carpet.

'How long have you known each other?' Megan asked.

Sherlock didn't look up at her. 'Who?'

'You and John.'

His eyes scanned nothing, thinking. 'A year or so.'

Megan raised her eyebrows. 'It's quite a while, isn't it?'

'No it's not.'

'I think it is; and in that amount of time, how many girls has John brought home?'

'Six. They all ended badly.'

'Right. And how many girls have you brought home?'

He paused. 'One. Sort of. She broke in. I don't see her anymore.'

'Uh-huh.'

'Why is this important?'

Megan shook her head. 'Oh, it's not at all. It's just...well, have and John ever-?'

Sherlock went silent before sitting up on his haunches, looking up at her. '...Have we what?'

'You know what I mean.'

Sherlock sighed inwardly. Megan shook her head and looked away. 'Sorry. It's not really any of my business.'

'No.'

'...I'm sorry?'

'No, we haven't.' He said, looking at the floor.

'Oh. Okay then.'

They both went silent again, before Megan said, 'but you want to.'

Sherlock looked up at her slowly. 'I beg your pardon?' He said, frowning.

'...No, noth-'

He stood up suddenly, towering over Megan. 'What gives you the right,' He hissed, 'to decide on my feelings for me?'

Megan looked a little taken aback. 'I'm only-'

'No. Just don't.'

'Sherlock.' John said, poking his head around the door. Sherlock looked at him, looked back at Megan coldly, and then followed John out of the room. Megan sighed to herself and followed them.

'We found something in the living room.' John said, leading the detective towards it. Sherlock stared, open-mouthed.

Scratched into the mirror hanging on the wall.

IOU.

Sherlock walked slowly towards it, tracing the letters with a gloved finger.

'You said he was dead.' John said eventually.

'...He can't be alive. He just can't.' Sherlock said in an almost whisper. He noticed something tucked on top of the mirror. He pulled the note out, unfolding it.

Sleep well, honey.

Sherlocks eyes suddenly widened. He darted out of the living room and into Vanessa's room with Megan, John and Keri following behind him quickly.

'What? What is it?' John asked as Sherlock dived under Vanessa's bed. He unclipped something that was fastened to the mattress and straightened up, holding it in his hands.

'Carbon monoxide canister. Designed to release the gas every time she went to bed. It's empty now.' He glanced up at Megan. 'What was she complaining of? Headaches? Nausea? The symptoms of carbon monoxide poisoning.'

Keri clapped a hand over her mouth. 'Oh my God...'

'I know. Brilliant work. Very clever.'

'Sherlock...' John said quickly.

He looked up at him. 'Hm?'

John shook his head slowly. Sherlock paused. '...Not good?'

'No.'