Friday, 21st of December, 2011. 7:00 a.m. in the morning. The news bulletin opens with the 10th death in a row. "London Metropolitan police are conducting an investigation after the body of a middle-aged woman was found fully clothed, lying prone on the ground of Walthamstow Central Station. At first, constable B.I., the first police officer arriving at the crime scene, speculated about the possibility of a natural death. She soon changed her mind after seeing the sheer terror in the woman's motionless face."

Paul looks at his kettle. It definitely needs descaling. The sound of boiling water muffles the radio receiver, making the news bulletin unintelligible. The toaster is on. He is about to breakfast on tea and toasts, as usual. Suddenly, his mobile phone rings. "What the hell?", he murmurs. He is chief in command for the special operation launched on account of the series of killings that have been taking place recently. He warned the team not to bother him unless a new corpse was found. "It cannot be the case, not again", he tells himself.

- "Inspector May speaking", Paul answers reluctantly.
- "It's me, Beth. Sir, following the protocol, the area has been cordoned off. The mayor is coming and has requested your presence. Walthamstow Central Station, right at the entrance.", by her hurried voice's tone and the location of the body, Paul knows full well that it is going to be a long day.
- "I'm on the way. Don't talk to the media. Don't talk to anyone!"
- "Yes, sir.", they both hung up in unison.

7:30 a.m. Paul arrives at the surroundings of Walthamstow Central Station and a noisy flock of curious people are already trying to poke their nose into the investigation. From his position, Paul can see Beth accounting to the major. He jumps to his feet and approaches the scene, clawing his way amongst the crowd.

- "Good Morning, mayor Palmer", Paul utters with a serious countenance.
- "Good Morning, inspector", Mr. Palmer says while gesturing to Beth to recapitulate the facts.
- "Inspector, all the officers are completely baffled by this murder. We couldn't have predicted this twist in the case. We managed to foresee the station but the assassination was expected on Sunday 23rd.", Beth pointed out.
- "Well, it doesn't make any difference, we're going to arrest the killer at any cost. Mayor, as you may've been informed, the first body was found in Elephant & Castle Station while I was holding a dinner to celebrate my retirement. I had to put off that dinner for a while. I promised to stay in command until the case is solved and I'll keep my promise. It's all I can tell you at this moment."
- "I understand. Keep me informed. Officer, inspector", mayor Palmer says as he beckons his bodyguard. He leaves the scene jostling through the swarm of prying people.

9:30 a.m. The Metropolitan Police's headquarters is a hive of activity. David, the computer technician remains silent whilst Beth relates the story. By his puzzled and thoughtful look, you can guess the news comes as a complete surprise to him.

The agenda for today's briefing consists of a unique item: under no circumstances should anyone leak any information about the current inquiries. The inspector ends the meeting with a muted applause and his favourite phrase: "Do I make myself clear?". The instructions are as clear as day.

The special operation command has been working hard since the first body was found 21 days ago. Nine deaths, all poisoned. Nine stations, each from a different line. But, things are seldom as they seem. David had requested an urgent meeting on Tuesday to announce an unexpected connection between the deaths. Unfortunately, the meeting, postponed until Wednesday, was broken up after the room's door burst open and constable Beth announced the 9th death. David was talking ten to the dozen, trying to shed light on the recent deaths. He looked at Beth and came out with a name. "Heathrow Terminal 5 station". Beth nodded at him in agreement. The news left the inspector speechless. A deathly silence fell on the room.

A few minutes later, David managed to resume the speech, giving detailed account of his findings. The connection between the string of deaths was purely and simply numerical. Looking at the calendar on the wall, David reeled off the first few numbers that were crossed out: 1, 2, 3, 5, 7, 11. It was a well-known sequence, and as yet no officer had discovered the link. Prime numbers. The team looked on in awe as David rendered the second discovery: the stations had not been chosen at random. They also followed a clear pattern: the last station of a line, the line chosen in alphabetical order. A two-slide Powerpoint presentation showed a couple of Wikipedia articles supporting the explanation.
Unfortunately, society often attributes people who suffer from a mental illness with aggressive traits. Say the words disorder and crime and our brain would end up making a strong connection that would eventually make us impossible to see the forest for the trees. I'm unable to hurt anyone. But appearances, more often than not, are deceptive. 

Back on her desk, Beth wonders why the killer has changed his pattern. It is unusual for a serial killer to make the slightest change in the modus operandi. "We must've overlooked something, this case must've been a coincidence", she mumbles. David is sitting next to her, apparently aloof. He wonders why the death has been advanced in time. It does not make any sense. Confused and stumped, they rack their brains to find an explanation for the happenings.

Beth and David make a thorough review of the events and after careful consideration they draw a conclusion: the 10th body does not belong to the case. It was sheer coincidence to find it in Walthamstow Central Station. They both know that there is little statistical evidence to support this "coincidence", however. Two printed sheets of paper rest on the workstation, showing a list of Underground lines and the Prime Numbers sequence.

01:00 p.m. Inspector May holds an online press conference. The news has crossed borders and nearly a thousand media are accredited to attend this event. Joana is the press officer in charge. She has advised Paul to be wary about unveiling too much information about the case. In short, he only said that Police had not ruled out any possibility. Nothing about David's insights.

07:00 p.m. Gathered around the desk table of Paul's office, a copy of the postmortem examination is handed out to Beth and David. She died of a heart attack. No traces of having been poisoned. It all seems to indicate that they were right in assuming the disconnection between this victim and the rest of the lot. Paul cracks a smile. Beth and David mimics him. No mystery surrounds the circumstances of her death.

9:00 p.m. Today's last briefing is conducted by constable Beth. She addresses the members of the special operation, following Paul's instructions. On Sunday, they will have to swarm Walthamstow Central Station with plainclothes officers. They will have to pay special attention to vending machines and snack and beverage stalls because all the victims have been poisoned so far. They will ask anyone that behaves suspiciously for identification.

Saturday, 22nd of December, 2011. 11:00 p.m. The station has been already occupied by plainclothes officers. Some regular police officers cover the surrounding area, otherwise, it would become suspicious after the fuss created around the case. People were in panic. In fact, the security measures have been reinforced throughout the network of tube stations. A group of officers alongside each entrance provides a sense of security among the citizens.

Monday, 24th of December, 2011. 7:30 a.m. The atmosphere at Metropolitan Police headquarters is unbearable. They have been wrong about yesterday's operation. No death, no arrests, no killer. Probably, the killer is aware of the police's findings and has given up. Maybe they will never catch the assassin. Paul is hopeless about solving the case. "23" is the tenth Prime Number and there is no death yet. "If only the killer had acted", he thinks amorally.

8:00 a.m. Paul receives the call. The mayor cannot wait any more. He has been put under extreme pressure both by citizens and political opposition. He must make an statement, he cannot bury his head in the sand. He has no option but to address the nation and explain the unsuccessful operation, revealing all the details. Paul's unshaven face shows an angry expression. His last case and he would end up as a complete idiot.

8:30 a.m. The press conference is scheduled for midday. Less than four hours left. David, the computer technician, is desperate and this situation is driving him mad. He cannot think clearly. His analitical mind does not allow him to work under pressure, except that he is about to solve the case. Looking at the picture of Friday's corpse, he realises that police have not investigated her digital footprint. David beckons Beth to his desk.

- "Beth, I have something to tell you, but you have to be prepared to act on a hunch. We know nothing about the discarded body. Only her name and age: Clara Hinder, 38. I have tried to locate her on Facebook but there is no account with such a name. Where are her belongings? Could we gain access to them?", he said hastily.
- "I also had the hunch that we were overlooking something, but the postmortem was conclusive and decisive. Wasn't it?", replied Beth.
- "You haven't answered my questions yet. I need her mobile phone!”

11:00 a.m. After many calls, Beth had almost lost her patience when she realised that if there were a person who could do her a favour, it would be Roger, an old friend and an expert on "police shortcuts". Roger makes an appearance and hands out a special authorisation to Beth.

11:30 a.m. David is trying to hack into Mrs. Hinder's mobile phone. It is impossible. The device could only be accessed by using a six-digit PIN number or the owner's fingerprint. After 10 failed attempts, the contents of the phone would be auto erased. "One hundred thousand possibilities and only ten attempts, it is impossible", he thinks aloud. Beth approaches David nervously.

- "I have overheard you. I think we have a chance."
- "I'm all ears, tell me about it."
- "If only we could hack her fingertip."
- "Go on, please."
- "The corpse is still inside our building."

David jumps up and runs through the office, heading downstairs into the gloomy room where postmortem takes place. The lifeless corpse remains restful, undistrubed. He turns on the lights and grabs the right hand. It is cold and numb. He places the forefinger on the phone sensor. It does not work.

11:45 a.m. In 15 minutes the mayor could jeopardise the whole operation. Beth and David look at each other in despair. They take a deep breath. David is still holding Clara's hand.

- "Try again, sometimes my phone sensor is a bit sluggish", she tells David.

Following Beth's advice, he repeats the process. A slight vibration and the phone is unlocked. Five percent battery left. Beth and David sit down on the ice floor, childishly. No Facebook account, no whatsapp app installed, no Twitter account, no e-mail address configured, no SMS sent or received in ages. A video of the victim is the only will. Unopened, unsent, unseen, virgin. They nod to each other.

12:30 p.m. The mayor press conference is over. All the nation is overwhelmed by the police findings. The murder could not act any more following this scheme. No more train stations, no more Prime Numbers. The mayor receives the honours. Paul ends up unhonoured.

Beth and David have just seen the video. Mrs. Hinder has just revealed the secret. The first body was the leader of a cult called "There is no Tomorrow". They claimed that the end of the world was expected to materialise on 31st of December 2011. Eleven members had been chosen by God to leave this world in advance. They would go to heaven, directly. Their followers did not know who has been elected until they received a videomessage from the last victim. Mrs. Hinder died of a heart attack. No traces of having been poisoned. No message handed out to the next victim, no more suicides. Beth and David will bury the 11th name, they will keep this video secret, forever.