Lambert, a scientist, has invented a highly explosive powder known as "Greek fire", which the Sheriff wants to exploit. But Lambert does not trust the Sheriff not to use it as a weapon, and refuses to hand over the recipe, thus reneging on his agreement with Guy of Gisbourne (who had commissioned him to make it.) The instructions for making "Greek fire" are written in Lambert's ledger (journal), and the Sheriff orders him to be thrown into the castle dungeons until he agrees to reveal where it is hidden. The Sheriff already has three barrels of the powder given to him by Lambert in advance.
When Robin finds out about the ledger, it is decided Much will get himself thrown into the castle dungeons so he can free Lambert. Much stands in the marketplace shouting insulting remarks about Prince John and the Sheriff. No-one takes any notice and he is only arrested after attacking one of the Sheriff's guards who have been listening indifferently. Much is dragged in front of the Sheriff before he can help Lambert. Gisbourne tries to persuade Lambert to reveal the location of the ledger, playing on their friendship, but is unsuccessful.
The Sheriff, recognizing Much as Robin's right hand man, decides to keep an eye on him and instead of hanging him appoints him as the Earl of Bonchurch. He also assigns a girl named Eve to the task of spying on Much and reporting back to the Sheriff. Meanwhile, Marian, in order to prevent the ledger falling into the Sheriff's hands, suggests a plan to Gisbourne. He will take advantage of the Sheriff's absence during tomorrow's Council of Nobles to rescue Lambert and have him brought to safety at Kirklees Abbey - but only if Lambert (who does still trust Gisborne, to some extent) hands over the ledger first. Gisbourne, who thinks the Sheriff will not care what he does as long as they get the ledger, agrees.
Robin breaks into the dungeons and asks Lambert to reveal where the ledger is. At first he will not cooperate, but Robin reminds him that he will eventually crack under torture and tell the Sheriff everything. Lambert says the ledger is hidden in a water barrel outside the castle and Robin, after swearing to keep it safe until he can help free Lambert, goes off to retrieve it. His men are excited at having found the ledger. Djaq is desperate to find out how the Greek Fire is made, as the Saracens have been experimenting for years and never managed to produce any. She wants to keep the book to study. Will says "This is not about one man's wish" (Lambert's wish to preserve the ledger) and suggests they destroy it. Later, Marian informs Robin that Gisborne should have the ledger by tomorrow, but Robin already has it. She suggests he lie in wait for Gisborne's men and "intercept" Lambert on the way to Kirklees. She also tells him to burn the ledger at once. Robin refuses to destroy it, as he promised Lambert he would keep it safe.
The Sheriff finds out that Robin knows the whereabouts of the ledger. When Gisborne returns to the castle, the Sheriff tells him about this act of "betrayal". That night, Lambert is tortured to death, with the approval of Gisbourne, who no longer feels any tie of friendship or loyalty to Lambert. Robin visits Much at his new estate and asks him to find out where the Sheriff's three barrels of Greek Fire are hidden.
The next day, Much attends the Council of Nobles. Marian is surprised to see that Gisborne is present and did not go to rescue Lambert, but he callously tells her Lambert is dead and no longer matters to him. The Sheriff ridicules Much in front of the other nobles but allows him to attend a celebratory feast that evening. Marian tells Robin that Lambert is dead, and cries on his shoulder.
At the nobles' banquet, Much seems to be getting along very well with the servant girl Eve, whom is suspected of spying on him under the Sheriff's orders. On seeing this, Marian takes him aside and asks him to tell Eve that the ledger is located in a powder barrel. Much agrees, but unknown to him, Eve has been listening at the door.
Eve and Much exchange personal talk about their lives and pasts while he soaks in a large bathtub. She persuades him to speak of the ledger and he says he is willing to tell her where it is, even though he knows she is spying for the Sheriff. He then suggests that she also take a bath. A resigned Eve starts to undress before a shocked Much insists that she wear a bathing robe and occupy another tub. Impressed by such gentlemanly behaviour Eve reveals that she was spying for the Sheriff, and was happy that Much was unwilling to go along with Marian's lie. In return for his loyalty to her, she promises to lie to the Sheriff about the ledger's location, and helps Much to escape back into the forest. Before he leaves, he kisses her and swears to return and find her when the Sheriff is gone and justice restored.
Marian, still furious about Lambert's death, has removed her engagement ring. She will only continue to wear it as long as Gisbourne ensures he will never put her in a situation like that again. Eve tells the Sheriff that the ledger is hidden inside one of the barrels of Greek fire. Gisbourne's men are sent to look for it, and are unknowingly followed by Robin. When Gisbourne returns empty handed, the Sheriff realizes that Eve led them into a trap. He is furious and rides to the mountain with Gisbourne. Unfortunately for the Sheriff and Gisbourne, Robin is waiting for him, and blows up the powder with a well placed Fire Arrow. Gisbourne is seconds from death when the cave blows up, and only just survives. A furious Sheriff tells Gisbourne to clean up the mess you've made!
Robin and his men discuss whether or not to destroy the ledger. Djaq wants to keep it but Robin, repeating what Will had said earlier, says "This is not about one man's wish", and throws it into the fire. While the men are distracted, she salvages the ledger from the flames, unaware that Robin is watching her. However, there is an open possibility that this was planned, as it appears Robin may have deliberately distracted the men so that the ledger could be saved.