The Sheriff and his sinister scientist Joseph are testing a chemical weapon on a street in Nottingham and blaming the ill-effects on the pestilence, but Will Scarlett's father Dan knows it's not the pestilence at all. When the Sheriff murders Dan for speaking out against a suspected poisoning, Robin must track down the source of poison in time. But what will he do when he realises that Will is intent on avenging his father's death?