“A carbon monoxide detector is like a spouse,” Agent Li declared while installing the shiny new device, “Always nagging when something’s wrong, but you’d be dead without them.” He chuckled at his own joke, though his colleague Agent Zhang only rolled her eyes.

“Just hurry up. Our target will be here in twenty minutes,” Zhang checked her watch for the tenth time. Their mission was simple: plant the modified detector in the study of Professor Chen, a suspected intelligence leak at the research institute.

“You know what’s funny?” Li continued, unfazed by Zhang’s impatience. “The professor studies gas chromatography. He should appreciate our little gift.”

“That’s not funny, that’s ironic,” Zhang corrected him mechanically. “And please focus on the installation.”

The detector looked innocent enough - a standard model available in any hardware store. But inside lay a sophisticated listening device, designed to pick up every word spoken in the study. Li had insisted on this approach, claiming its dark humor made it his masterpiece of spy craft.

“Done!” Li stepped back to admire his handiwork. “Beautiful, isn’t it? Like a white mushroom growing on the ceiling.”

“Comparing surveillance equipment to fungi? You’ve been reading too much Wang Xiaobo,” Zhang muttered, but a slight smile betrayed her amusement.

They cleaned up quickly and left through the back door, just as Professor Chen’s car pulled into the driveway. Phase one complete.

Over the next week, the detector performed its dual duty admirably - both protecting the professor from carbon monoxide and feeding his conversations to the agency. But Li couldn’t shake a nagging feeling about the whole operation.

“Something seems off,” he told Zhang during their monitoring shift. “The professor’s conversations are too perfect, too incriminating. It’s like he’s reading from a script.”

“Maybe he’s just not as clever as we thought,” Zhang suggested, but Li noticed she was frowning too.

On the ninth day, during what should have been a routine check of the audio feed, they heard the professor speaking directly to the detector:

“I must compliment whoever installed this delightful device. Very elegant work. But you should know - I specialized in miniature gas sensors before switching to chromatography. Did you really think I wouldn’t notice?”

Li and Zhang exchanged alarmed looks. Before they could react, the professor continued:

“I took the liberty of making some modifications. You see, your device had space for an extra component. A small canister, releasing trace amounts of carbon monoxide. Not enough to trigger the alarm, but enough to cause interesting effects over time. Headaches, confusion, poor judgment… I do hope you’ve been monitoring the feed in person.”

The room started spinning. Li grabbed the desk for support, finally understanding the dizziness he’d been feeling during his shifts. Next to him, Zhang slumped in her chair.

“Karma,” Li managed to whisper, appreciating the dark humor even as his consciousness faded, “is a lot like a carbon monoxide detector too. Silent until it’s too late.”

The last thing he heard was Professor Chen’s voice through the speakers: “Don’t worry, the dose isn’t lethal. Just enough to ensure you’ll both be too compromised to testify at my trial. Good day, agents.”