The City of Animals never went to sleep. Acorn cars criss-crossed Moose and Lynx Streets, and lamp posts cast orange light across the wide pavements. On the tenth floor of a high-rise overlooking the park, two young foxes - twins Lena and Frederick - had their lair. For as long as they could remember, they had always stuck together, seeking adventure where others saw only boredom. It was a cold and windy November night when Frederick spotted something strange during his evening patrol around the estate. "Come quick!" - he hissed to Lena, jumping up and down with excitement. "Check it out!" On the damp pavement, just up against the wall of an old, abandoned hamster food factory, there was a trail of narrow, shiny footprints. They looked as if they had been left by tiny paws, but... they were smudged and irregular in places, as if the animal that had left them had trouble moving. "I don't think these are dog tracks," Lena said, looking at the glare in the lamp posts. "Maybe some sick rats? But something's not right here... see, they're bigger than normal rat prints." Frederick leaned lower and lower, trying to pick up the scent. Instead of the typical rodent musk, he smelled something disturbing - a slightly metallic smell that resembled....... blood! The fox squirmed slightly and looked meaningfully at his sister. "Shall we go?" asked Lena, although her voice was no longer as sure as usual. They looked at each other for a moment, then together they crossed the row of yellow tape with which the police had long ago cordoned off the entrance to the factory. Inside, there was a semi-darkness that was cut to pieces by the lantern light coming through the dirty windows. Their every step echoed among the empty halls. Footsteps led straight to the great, rusty doors of the warehouse. Frederick was already about to open it when suddenly they heard a quiet, trembling rustling from behind them.... Lena held her breath. The door began to slowly swing open from the inside....