The Sheriff and his sinister scientist, Joseph (Ralf Little), are testing a chemical weapon on a street in Nottingham Town and blaming the ill effects on the pestilence, but Dan Scarlett, who is in town to visit his son, Will, knows it is not the pestilence at all.
Dan wants Will to become his apprentice and run the family's carpentry business, but Will is frustrated that his father is blind to the good work he does as an outlaw. Father and son bicker about what is best for Will, but tragically fail to resolve their dispute before it is too late. Dan speaks out against the Sheriff, because he knows it is poison and not the pestilence, and he does it on behalf of Will telling him to stand up for himself. The Sheriff murders Dan for spreading the truth, leaving Will and his younger brother, Luke, heartbroken.
Now Robin realises that the Sheriff is up to something and resolves to track down the source of the poison before more innocent people die. Bravely plunging into the heart of "pestilence" territory, the outlaws work tirelessly to soothe the sick, little realising that the strange helper from Leicester is none other than Joseph, who is recording the results of his "experiment."
Worse still, Robin believes Will has returned to Scarborough to be with his brother, when, in fact, he is headed straight for the Castle to punish the Sheriff for his act. With lives at stake, Robin must track down the source of the poison in time, but what will he do when he realises that Will is intent on avenging his father's death at any cost?
Little John discovers a cure for the poison, an old woodman's remedy. With the sick on the mend, the outlaws head to the castle to find Will has already poisoned the Sheriff and Joseph. After making the Sheriff apologise to Nottingham for trying to poison them, Robin convinces Will to use the antidote on the Sheriff.
Angry that there's a cure to his poison, Joseph lunges at Robin, then accidentally falls to his death out of a window of the castle. Cheating a fall too, Robin and the outlaws return through the town to the cheers of Nottingham.