When a group of boys accidentally stumbles on Gisborne's weapons-testing site, all but one is taken prisoner. The resourceful escapee, Daniel, is able to elude Gisborne's men and runs to Robin to raise the alarm. Robin is horrified to discover that Gisborne has developed an indestructible new suit of armour, made of special Damascus steel – an army wearing this armour would easily defeat King Richard and must be stopped.
Gisborne has taken the boys back to Locksley Manor where the Sheriff orders that they be hanged. Luckily, Robin and his gang arrive in the nick of time. What's more, Robin cleverly steals the impressive black diamond, used to create the Damascus steel. But disaster strikes and Daniel is captured during the scramble.The Sheriff immediately demands an exchange – Robin can have Daniel back if he hands over the black diamond. Robin's priority is rescuing Daniel and he and the gang decide that they will agree to the exchange and, once made, they will then set light to the diamond's box and melt the black diamond once and for all.Marian learns of Daniel's capture and puts a plan into action. Feigning interest in seeing Gisborne, she requests a night pass from the Sheriff. She doctors the document so that the signature can be mistaken as a release for Daniel. Marian runs her plan past Robin as she needs one of the outlaws to pose as a guard and take Daniel. To everyone's surprise, Allan volunteers, but he only sees it as a brilliant opportunity to leak Robin's plan to Gisborne.