Penny straightened on the yoga mat, moving from a low runners pose into a high lunge, rolling her shoulders back and putting her hands on her hips, pressing her heel firmly into the mat. She had the lithe muscular build of an athlete that she worked hard to maintain, her daily routine always featuring a run, yoga, swimming. Activity gave her some peace, though it often reminded her of the shrapnel scars on her abdomen from the battle that took her left arm. Ghost sensations from memories made her prosthetics fingers clench as she stretched.

On the other side of the room, Sheldon was multitasking, hyperactively lurching between three white boards, his computer and the TV. Two boards were work on theoretical physics, the other he'd said was a "bit of fun with the P=nP problem" but the television now had his attention.

He froze stock still for a good five seconds, frowning at the screen, then said "Penny, we'll be getting a call very soon."

She sat down on the mat and reached for her towel.

"What makes you think that?" she asked as she towelled her neck.

He Pointed to the TV, where the news anchor cut to helicopter footage of highway where a mass of police cars and ambulances with flashing lights surrounded a yellow school bus that had run off the side of the road,

He answers calmly. "A breaking news story, they have the traffic helicopter doing on the scene footage. No signs of high speed impact or skid marks on the road, no obvious damage to the bus, yet massive police, fire and medical response and from multiple police elements – they didn't know if it was a traffic case, a violent crime, a theft or what so they sent everything."

"Well, that kind of proves it involves the kids, it's a normal adult overprotective reaction"

"That level of early response and the news thinking it newsworthy says something bad has happened to those children, something Unusual"

"and they come to you for the strange and unusual"

"of course. "

"Think it'll be dangerous?"

"Oh, most definitely. I have a feeling this is something special"

Penny closes her left hand into a fist "well, I'm fully charged. I'll get changed and arm up"

Sheldon's phone rings. Penny jumps to her feet and heads for her room as he answers

"Ah, Hello Detective. We're ready to go, is the car on its way?"

Penny quickly changed into jeans and a casual shirt over a vest top, long sleeved as she was still self-conscious about her arm and long enough to cover the pistol she carried in a belt holster in the small of her back.

The magazines were single stack, so slim enough for her to carry a spare in her right hand pocket. They were men's jeans as she loved the pockets, though she'd had to have the waistband taken in to fit.

In the car, Sheldon was rapidly scrolling thought school bus facts when Penny asked "So, what do we know?"

"Well, School buses are something of a rarity in California, as it is one of the few states that does not require schools to provide buses. With so many children either living near school or having a parent who drives to work it's a very small and specific demographic who catch the bus"

"So it'll be kids who go to school out of walking distance, from families who have no car, no parents or parents who work weird hours."

"indeed – so we can rule out kidnapping to blackmail the families for money"

"I just hope no one was hurt.."

At the scene Detective Hoffsteader was waiting, a small perpetually hassled looking man with a face like a tired apology. He hurries over from the crowd of police and emergency services, pausing to straighten his hair when he sees Penny get out of the car.

"ah, Detective! What do we have?"

"Ten young children missing from a school bus that just stopped. The driver says she must have fallen asleep at the wheel, drifted off the road and stopped on the shoulder. The Kids all say it was like a funny dream, not sure what happened but say a "strange man" came and took the youngest kids"

"A strange man?"

"That's the best description we could get. Something about a man with a head like a big red balloon. Then they all started screaming. Emergency medical and childrens services are trying to help them"

"Curious"

Sheldon walked up to the Ambulance where the bus driver was sitting, a middle aged coloured woman wrapped in a silver blanket but shivering uncontrollably.

She looks up at him with a tear stained face and stumbles "I don't know what happened. I was just driving along…"

He cut her short by clicking his fingers three times in front of her face, staring at her intently. Then he makes a tutting noise and rapidly walks to the bus.

Penny shakes her head and sighs, then sits next to the driver, taking the womans hand. "It's Ok, we're here to help. We're going to find out what happened"

Sheldon produces a pair of surgical gloves and a face mask from his bag and puts them on.

Then he boarded the bus, eyes darting as he surveyed the scene. He dropped to the floor, observing the rubberised floor cover. In several places he finds scattered crumbs of stale bread, pointing them out to be photographed and sampled by forensic techs.

By one of the seats he finds a dropped juice box, half crushed. In the sticky residue a large boot print is clear.

Then he darted to the driver's seat, looking at the wheel, the padding of the chair, the gear shift. He sniffed, taking in the abhorrent mix of scents from an old bus of young kids on a hot day.

A sudden realisation dawns and he strides purposefully back to Penny who was still trying to comfort the driver.

"I have a theory. But first, Penny, in your professional opinion how is she?"

Penny stands, reassuring the driver then leads Sheldon a few steps away and whispers.

"Traumatised but not handling it normally. I've seen my fair share of traumatic stress and this is just wrong. She's both calm and panicking at once, massive adrenal comedown with shakes and double vision yet she's all calm and dreamy"

"Look at her pupils"

Penny pulled a pen torch from her pocket and softly asked the driver "Could you look at me? OK, then over there? Now follow my finger" She nods and turns back to him.

"Sheldon, she's definitely under the influence of something. Pupil response is way off."

"That's what I expected"

He leads Penny to the bus. "Now, smell the driver's seat, the side near the wall."

Penny looks puzzled but sniffs tentatively. Old upholstery, sweaty days of driving. But she sniffs again, frowning at she catches something faintly familiar, a sweet medical scent that reminds her of hospital wards.

Sheldon nods "you noticed it? That smell is an aerosol sedative, most likely Haldol or a derivative."

Penny holds up her hand to stop him "No, not Haldol. it's similar, but I think it's Sevoflor. I remember it from surgery"

"Sevoflourane? That fits. It's the preferred anaesthetic for children"

He reaches over and carefully takes down the little fire extinguisher cylinder from the wall, examining it closely.

"This is the right age and model for this bus, but this one has been removed and replaced recently. See the clip has chipped the paint and the metal revealed is still fresh and bright?"

He puts the cylinder in a plastic evidence bag

"Still, just dosing someone is too random to rely on… unless"

He marches back to the driver. "You've had a mandatory medical recently, haven't you?"

She jumps, then nods "Yes, two weeks back. How did you know? They called me in; it was for my insurance to stop my premiums going up"

Sheldon nods. "That allowed them to more accurately estimate dosage and flow rate"

Penny pulls out her phone "I need to talk to the medics treating the kids – it's emergence agitation, they're panicking as the sedative wears off"

Detective Hoffsteader walks over and asks "So, what do you think?"

Sheldon holds up the evidence bag "Sometime in the last few days someone swapped the buses fire extinguisher for a duplicate they had made which was filled with an aerosol sedative and had a remote trigger. Probably radio, a timer is too random in this traffic. When activated, the device sprayed an invisible stream of hypnotic sedative gas onto the driver, who breathed it in and slowly slipped into unconsciousness. The process was gradual enough that the driver felt sleepy and could pull over before she passed out. The gas also spread down the bus, calming the children and making them sleepy.

Then whoever planned this boarded the bus and took the youngest and smallest children, most likely as they'd have been the most effected by secondary exposure to the sedative as they were smallest and sat at the front."

Penny adds "That shows care, at least. The ones they took would need medical care to avoid complications or overdose."

Sheldon shrugs "It also hid what they had done better. And smaller are easier to carry, They also swapped the fire extinguisher back, taking their fake one with them"

Detective Hoffsteader frowns, wrinkling his bushy eyebrows

"But how can you take ten kids off a bus without anyone stopping you?"

"That much is simple, Detective. They put them on their own school bus" Sheldon holds up his phone, showing a second hand auto site. "You can buy an old school bus for a few thousand dollars"