Mongrels looks at the lives of five animals that hang around the back of a pub called The Lord Nelson on the Isle of Dogs in the East End of London. Nelson is a fox who lives a metrosexual lifestyle (Vulpus metrosexualus). Described as: "The only wild fox in East London with subscriptions to all the major broadsheets (excluding The Sunday Times), Nelson is, as he never tires of introducing himself at dinner parties... 'An urbane fox!'"
Nelson's love interest is Destiny (Canis self-absorbedbitchicus), an Afghan hound. However, Destiny has no interest in Nelson. She is the pet dog of Gary, the landlord of The Lord Nelson. However, like the other humans that appear in the series, he cannot understand what Destiny or what the other animals are saying. Elsewhere there is Marion (Felis retardicus), an idiotic cat to whom Nelson acts as a father-figure. Marion has been abandoned by several owners and is very corruptible. Then there is Kali (Aves aggravaticus), a pigeon who likes to revel in the misfortune of others. She has several grudges, including a hatred of all humans and foxes for the way they treat birds. Lastly, there is Vince (Vulpus c***itcus), a unhappy, foul-mouthed fox who considers himself a proper animal.