Two criminal brothers with a grudge against Inspector Rivera kidnap Rex and inject him with a pheromone to make him aggressive, and then use him in a series of robberies. Rex is now considered dangerous and the police are ordered to destroy him. Rivera is desperate to find Rex before the police and try to save him.

Inspector Rivera and Rex are after an elusive criminal called 'The Grey', but must compete with a rogue policeman Paolo Miano, who is also looking for the offender, seeking revenge for the death of his brother. Their efforts are thwarted by a mole who is always one step ahead of their moves.