Friday tried the door handle and it opened. ‘You don’t worry about being axe murdered in your sleep?’

  ‘The incidence of axe murdering is statistically very low,’ said Mum. ‘Whereas the incidence of older citizens dying of cold-related illnesses is surprisingly high.’

  When Friday stepped into the apartment, she was shocked. She knew her mother was a terrible housekeeper, but this small two-bedroom apartment contained a quantity of filth so large, even a whole fraternity house full of engineering students would struggle to match it. There were notes, periodicals and takeaway containers littered everywhere.

  ‘Was it this messy when you were arrested?’ asked Friday.

  ‘No,’ said Mum.

  ‘So the police did this when they searched the place?’ asked Friday.

  ‘No, there was more junk,’ said Mum. ‘The police took a lot of it as evidence.’

  ‘Wow, Dr B,’ said Melanie. ‘I have four brothers and even they would struggle to generate this much mess.’

  ‘Perhaps you could help me with it,’ said Mum.

  ‘I don’t have access to a bulldozer,’ said Friday.

  ‘Not the symptom,’ said Mum. ‘The cause.’

  ‘Surely the cause of this mess is your laziness,’ said Friday.

  ‘No,’ said Mum. ‘It’s a mystery.’

  ‘It really isn’t,’ said Friday. ‘You’ve always had a total disregard for basic hygiene.’

  ‘No, it really is a mystery,’ said Mum. ‘And you solve mysteries, don’t you?’

  Mum peered at Friday as if checking that she was talking to the daughter she thought she was talking to.

  ‘Yes, I’m the detective one,’ said Friday. Friday was actually strangely touched. She hadn’t thought her mother was aware of what she did.

  ‘Then you can help,’ said Mum. ‘It’s my cleaner. She’s gone missing.’

  ‘Did you report this to the police?’ asked Friday. ‘That’s really serious.’

  ‘No, I didn’t realise at first,’ said Mum. ‘She comes by every day to clean up and cook me a meal. I thought she was gone on holiday. It was really very inconvenient. She didn’t ask for a leave of absence. I had to find my own food.’

  ‘I wondered why you were eating in a restaurant,’ said Friday. ‘That’s out of character for you. You don’t usually care what food tastes like.’

  ‘Such a waste of time,’ said Mum. ‘I had to walk there. I had to wait while they cooked it. I had to walk back. It was insufferably inefficient.’

  ‘So why didn’t you report that she was missing,’ said Friday.

  ‘I don’t have time for such trivialities,’ said Mum.

  ‘If someone is missing, that’s not trivial,’ said Friday. ‘At least not for the person involved. What was her name? Where did she come from? Who hired her for you?’

  ‘Connie,’ said Mum. ‘She wasn’t a terribly good cleaner. But she did work long hours. I suppose that’s something.’

  ‘Where did you get her?’ asked Friday.

  ‘What?’ said Mum. ‘She came to the apartment. You can’t clean an apartment without coming to the apartment.’

  ‘No, I mean, how did you hire her?’ asked Friday. ‘Did you use an agency? Have you contacted them to ask where she is?’

  ‘No,’ said Mum. ‘The institute organised her.’

  ‘Did you contact the Human Resources department and ask them?’ asked Friday.

  ‘No,’ said Mum. ‘I’m a busy woman. Dark matter doesn’t describe itself, you know.’

  ‘I’ll call them,’ said Friday. ‘Do you have a phone?’

  ‘Of course not,’ said Mum. ‘Phones destroy concentration. It’s impossible to maintain an appropriate research environment with one in the room.’

  ‘I’ve found one,’ said Melanie, pushing aside a large stack of papers. A landline handset was tucked sideways down the back of the couch. Melanie picked it up. The cord was dangling. ‘We just need a socket to plug it into.’

  Friday spotted one in the kitchen. ‘Over there.’

  They plugged the phone in, and Friday navigated the pre-recorded menus until she was put through to a human in Human Resources.

  ‘Yes,’ said the person on the line. From that one word she already sounded tired and irritated.

  ‘Hi,’ said Friday. ‘I’m with Dr Barnes in the Beaumont Building. The cleaner who services her apartment has gone missing. She’s a woman called Connie. We’re unsure of her surname.’

  ‘The institute does not provide cleaners,’ said the woman.

  ‘But a cleaner has been coming here five days a week to tidy up after my mother,’ said Friday.

  ‘The institute does not hire cleaning staff for the residential buildings,’ said the woman. ‘It’s the responsibility of the tenant to keep their own domicile clean. If they want to hire a cleaner, that’s for them to organise themselves. But the cleaner would have to get security clearance from us, and Dr Barnes has made no such request.’

  ‘But a woman has been coming here at nine o’clock every morning five days a week for the last three years,’ said Friday.

  ‘That is highly irregular,’ said the woman. Friday could hear her tapping away at her computer. ‘To enter the building you have to be scanned by face-recognition software, so we have a record of everyone who enters and when they enter. I’ll check the logs.’

  Friday waited while the woman looked up this information.

  ‘Okay,’ said the woman. ‘I can see that there is a woman who enters the building between eight fifty and nine every weekday. But she’s not a cleaner.’

  ‘Who is she?’ asked Friday. Perhaps her mother was meeting with a spy.

  ‘Corinne Smith,’ said the woman.

  ‘Okay, I’m going to put you on speaker so we can get Mum’s input on this,’ Friday turned to her mother. ‘Mum, are you sure you got your cleaner’s name right? Was it Connie or Corinne?’

  ‘I don’t know,’ said Mum. ‘I’m sure she said Connie. But it hardly matters.’

  ‘People’s names matter to them,’ said Friday.

  ‘Have you found her?’ asked Mum. ‘That’s splendid because this place could do with a tidy. I’m not fussy about appearances, but she really has let it go over the last couple of weeks.’

  ‘She has been missing,’ said Melanie.

  ‘Also,’ said the woman from the other end of the phone, ‘she’s not a cleaner.’

  ‘She’s not?’ said Friday.

  ‘Corinne Smith is a postgraduate student working on her physics doctorate,’ said the HR woman. ‘Dr Barnes is her academic mentor.’

  Friday groaned. ‘Mum, have you been using your students as servants again?’

  ‘Look, I don’t know their job titles,’ said Mum. ‘They’re cleaners. I don’t know why they have to be so precious about their job titles.’

  ‘She’s not a cleaner,’ said Friday. ‘She’s a research scientist.’

  ‘If you say so,’ said Mum.

  Friday took the phone off speaker. ‘I’m sorry we’ve wasted your time. But do you know where Ms Smith is? I hope my mother hasn’t driven her away from her career in science.’

  ‘No, she got a transfer three weeks ago,’ said the HR woman. ‘She moved over to the BETA experiment.’

  ‘And that’s good?’ asked Friday.

  ‘Oh, yes,’ said the woman. ‘It’s a promotion. She had lodged three official complaints about Dr Barnes. Moving her and giving her a pay rise would have been easier and more affordable than trying to get Dr Barnes to take empathy training, again.’

  ‘Thanks for your help,’ said Friday before hanging up the phone. She turned to Melanie. ‘I’m sorry.’

  ‘What for?’ asked Melanie.

  ‘We have to spend the next two weeks living in this squalor with my mother,’ said Friday.

  ‘It’ll be fun, like camping,’ said Melanie. ‘And with less chance of bear attacks.’

  ‘Let’s check out the spare room,’ said Friday.

  Dr Barnes had already sat herself down at her desk and was working away at something on her computer. She had disappeared into her own world.

  Friday pushed opened the door to the second bedroom.

  ‘Gosh,’ said Melanie. Both single beds, the bedside tables and the entire floor were all covered in stacks of periodicals.

  ‘Well, at least the mess in here is more organised,’ said Friday.

  ‘What are we going to do with it all?’ asked Melanie.

  ‘We could burn it in a dumpster fire,’ suggested Friday.

  ‘I’m pretty sure that would be immoral,’ said Melanie. ‘It would be burning literature and emitting carbon.’

  ‘If we don’t get rid of it, we’ll have nowhere to sleep tonight,’ said Friday.

  ‘Ms Dekker said the apartment across the corridor was empty,’ said Melanie. ‘We could relocate it all to there.’

  ‘I’d have to pick the lock,’ said Friday.

  ‘You love doing that,’ said Melanie.

  ‘We can’t leave it there,’ said Friday. ‘That would be dumping rubbish.’

  ‘Oh no, of course not,’ said Melanie. ‘We’ll just temporarily relocate it there. We can find a method of recycling it later. But we’ve had a busy day and I’d like to get some sleep.’

  ‘Okay,’ said Friday. ‘I’ll pick the lock. I saw a gardener outside when we arrived, you go and charm him. See if he’ll lend us a wheelbarrow.’

  Clearing the spare room took less than an hour. Properly cleaning the whole apartment was going to be a big job. They decided to go to bed and tackle that in the morning. Friday didn’t like cleaning, but she was glad they would have something physical to do. It would take her mind off worrying about Ian and where he could be.

  The next morning, Friday and Melanie went out early to get groceries. The only thing in Dr Barnes’ cupboards was an empty jar of instant coffee and a mouldy bottle of pasta sauce. An hour later, they were wandering back towards the apartment lugging several bags of shopping each. It was a beautiful blue day. It was cold, but the sky was so clear it was nice to be getting sun on their faces. The campus of CERN was a lot like a university campus. It was a clutter of drab functional architecture that belied the ground-breaking experiments being worked on by some of the greatest minds in the world behind those shoddy, weatherboard walls. Given that thousands of people worked at CERN, it was surprising how few people were visible. Friday supposed they were all more or less like her mother – so focused on their work, it wouldn’t occur to them to take advantage of a rare mild sunny day.

  ‘I can’t believe we’re stuck here for two weeks babysitting my mother,’ said Friday.

  ‘Switzerland is a pretty nice place to be stuck,’ said Melanie.

  ‘Really,’ said Friday. ‘There’s nothing scenic about an office complex.’

  ‘No,’ said Melanie. ‘But, look, there’s a helicopter.’ Melanie pointed to a distant dot in the sky. ‘That means there must be a helipad nearby. The ski slopes would be only minutes away by helicopter.’

  ‘We can’t keep an eye on Mum if we’re skiing,’ said Friday.

  ‘We can if she comes with us,’ said Melanie. ‘She’d love skiing. It’s all about physics. Gravity, friction, acceleration – all those things are involved.’

  ‘She’s a theoretical physicist,’ said Friday. ‘She’s not interested in real-world experimentation. At least, not herself. That’s for experimental physicists. Theoretical physicists view experimental physicists as being the intellectual equivalent of a pool cleaner.’

  ‘I’ve known some very lovely pool cleaners,’ said Melanie.

  ‘You’re thinking of human pool cleaners,’ said Friday. ‘Mum would think of an experimental physicist as having the intellectual capability of a motorised pool cleaner.’ Friday paused and looked up at the helicopter. It was much closer and getting closer still. ‘It looks like that helicopter is coming here.’

  ‘Excellent,’ said Melanie. ‘Then CERN must have its own helipad. Even better! I’ll ask Daddy if he knows any local pilots.’ Melanie’s father owned an airline so he most likely would.

  ‘Why would a helicopter be coming here?’ said Friday. ‘This facility runs experiments that take place over years. Everything is planned meticulously. There’s never anything urgent about it.’

  Friday had to stop speaking. The helicopter was now so close it was too loud to talk over. Melanie looked around behind them. ‘I don’t think there is a helipad. I think it’s going to land on that field.’

  There was a large lawn opposite the residential building. It looked like a park, but it was probably earmarked to be the site for another residential building eventually. Friday instinctively began to duck her head. The helicopter was still a couple of hundred metres away, but they could feel the down draft and ducking was what people always did in movies when helicopters approached. It passed right over their heads and landed in the middle of the lawn.

  ‘Perhaps it’s an important world leader,’ yelled Melanie.

  The door of the helicopter opened, and the girls soon saw for themselves. A large scruffy man awkwardly slid open the door and scrambled down from his seat.

  ‘Uncle Bernie?!’ cried Friday.

  He couldn’t hear her, but he had obviously seen them, because he ducked too – then ran over to the girls as the helicopter’s engine powered down and the rotor blades slowed.

  Friday was always happy to see her uncle. He was such a comforting presence. He was the only member of her family who had anything close to normal social skills. Plus, he was big and burly, so nice to hug.

  ‘What are you doing here?’ she asked happily.

  ‘Trying to have a conversation with my annoying niece,’ said Bernie. He clearly wasn’t in an equally good mood.

  ‘What have I done?’ asked Friday.

  ‘Where’s your phone?’ asked Bernie. ‘I’ve been trying to get in touch with you.’

  ‘We turned our phones off and left them in a locker at the Richelieu library in Paris,’ said Friday. ‘We didn’t want anyone tracking us.’

  ‘That meant I couldn’t track you!’ said Bernie. ‘I’ve been worried sick.’

  ‘About what?’ asked Friday.

  ‘I can’t find Ian,’ said Bernie.

  ‘What?’ said Friday. ‘But he was on the train. He can’t have gone far. He must have got off just before or after us.’

  ‘I’ve contacted every stationmaster at every station along the line,’ said Bernie. ‘They’ve scoured CCTV footage. They’ve put up posters with his photo on every platform. No-one has seen him. He disappeared. Or perhaps – someone disappeared him.’

  ‘No!’ said Friday. ‘But who would want to hurt Ian?’

  ‘I don’t know,’ said Bernie. ‘Perhaps someone found out what he does for Interpol.’

  ‘Or maybe he just went skiing,’ said Melanie.

  ‘What is it with you and skiing?’ asked Friday.

  ‘I can literally see the snow-capped mountains,’ said Melanie. ‘It’s hard not to think about it.’

  ‘But he wouldn’t just take off without telling us,’ said Friday.

  ‘He must be somewhere. Six-foot-tall, blond sixteen-year-olds don’t just disappear into thin air,’ said Bernie.

  ‘What did you say?’ asked Friday.

  ‘He can’t have disappeared into thin air,’ said Bernie.

  ‘No, the bit about him being tall and blond,’ said Friday. ‘What photos have you posted over all the train stations?’

  ‘The most recent one I could get,’ said Bernie. ‘I wanted a picture of what he was wearing. So I used an image from the CCTV footage at the Richelieu.’

  Bernie took his phone out of his pocket and showed them a picture of Ian. He was striding down the front steps of the Richelieu library, his blond hair flopping into his eyes and his navy-blue greatcoat perfectly tailored to his shape. He looked like a model in a Ralph Lauren catalogue.

  ‘Ian doesn’t look like this,’ said Friday.

  ‘What are you talking about?’ said Bernie. ‘I know what my own stepson looks like.’

  ‘Look at me,’ said Friday.

  ‘I didn’t like to say anything,’ said Bernie. ‘If you’re going through one of those awkward teenager phases, I’m sure it will pass.’

  ‘I’m not hormonal,’ said Friday. ‘I’m in disguise. So is Ian. I bleached my hair and he dyed his. He doesn’t look anything like this photo. He’s wearing a high-vis puffer coat, workman’s cargo pants, and steelcapped boots.’

  ‘And his hair has been dyed brown,’ added Melanie.

  ‘What?! I’ve had the entire French and Swiss rail service and police service looking for a tall well-dressed blond boy,’ said Bernie.

  ‘Lucky them,’ said Melanie.

  ‘Oh no,’ said Bernie. ‘I’m going to have to make some calls.’ He turned around and jogged back to the helicopter.

  ‘When did our lives get so complicated?’ asked Friday.

  ‘For me, it was the day I met you,’ said Melanie.

  Two days later, Dr Barnes was at her desk working, and Friday and Melanie were sitting on the lawn outside the residential building sunbathing. They had finally finished cleaning the apartment and exterminating every last adult cockroach. (The eggs would take years to get rid of, but they wouldn’t hatch until spring, so that wasn’t Friday’s problem.) For the first time in a long time, Friday could sit and relax.

  It’s hard to sunbathe in winter because you do need to wear a full coat. But, after weeks in the built-up city of Paris and two days scrubbing, de-moulding and cockroach bombing Dr Barnes’ apartment, it was lovely to be lying on sun lounges in the crisp Swiss air. They had a view of farmland spread out before them. The sun felt good, even if the wind was a little cold. Melanie was predictably fast asleep. Friday was reading a book, Large Hadron Collider Phenomenology by Kramer and Soler.

  It was weird to be tasked with babysitting your own parent. Friday had never babysat an actual child but, from all the Hollywood movies she’d seen and teen romance novels she’d read, she gathered this was what babysitting felt like. A lot of sitting round doing nothing while the person you were taking care of was in another room not dying.