The rifle had been hard to come by but it was a beautiful weapon. A Mosin-Nagant bolt action sniper rifle fully restored and operational, imported at great expense from Russia. It was standard issue for Soviet soldiers at the beginning of the Second World War and had been adapted in the thirties as a high-precision assassination device. After months of negotiation, the weapon had been delivered four weeks earlier and, in that time, the owner had become a competent shot.

This was fortunate. The rifle’s acquisition had initially been for the purposes of self-preservation but, with the change of plan coming about as a consequence of the traitor’s actions, the gun would play a central role in the solution.

On this occasion, however, the rabbit had experienced an episode of extreme good fortune; something had disturbed the creature’s grazing and it had scurried off to safety.

Displeased, the owner packed the rifle away in its carry-case and skulked up the basin side, through the garden and into the house. The owner placed the rifle on the kitchen table and proceeded to inspect the safe in the lounge.

It had occurred to the owner many years previously that the safe was a potential killing machine, although it was only recently when watching a rerun of the film Apollo 13 that the cause of death had become clear. The owner had previously assumed that the cause of death would be hypoxemia, oxygen deficiency, but instead it would be CO2 retention, as was the problem for the Apollo 13 astronauts.

Survival times were important to the owner. The rule of threes is of particular importance: three minutes without air, three hours at extreme temperatures, three days without water, three weeks without food.

Infuriatingly, there were difficulties calculating the prospective survival time in the safe given the number of academic disagreements regarding the value of certain variables.

The safe was a perfect cuboid, one point six metres in height, depth and width, giving just over four thousand litres of air. Air comprised around twenty-one per cent oxygen and less than half a per cent carbon dioxide. On average, a human generates thirteen litres of carbon dioxide and consumes fifteen litres of oxygen per hour, although other claims put this as high as eighteen litres an hour. There was some difference of opinion noted in the owner’s research about a fatal level of carbon dioxide but all accounts agreed that anything over six per cent was considered toxic. However, being optimistic about the chances of survival inside the airtight safe, the owner had used eight per cent as a definite lethal level, which equated to approximately three hundred and twenty litres. At a production rate of thirteen litres an hour, the owner calculated that a human would last around twenty-four hours before they would die from hypercarbia, or carbon dioxide poisoning. At a rate of eighteen litres per hour, the survival time dropped to eighteen hours.