In a sun-dappled Sherwood forest, the Doctor discovers an evil plan from beyond the stars and strikes up an unlikely alliance with Robin Hood. With all of Nottingham at stake, the Doctor must decide who is real and who is fake. Can impossible heroes actually exist?
Clara Oswald insists that the Doctor take her to meet Robin Hood, despite the Doctor’s claims that Robin Hood is only a legend. When the TARDIS lands in medieval England, they are met by a man who claims he is Robin Hood. Robin challenges the Doctor to a duel and the Doctor accepts, using a spoon instead of a sword. The Doctor manages to push Robin into a river, only to be pushed in by Robin later. Still, the Doctor remains in disbelief even as Robin takes the two to meet his band of Merry Men. Clara enjoys her time there and learns that Robin is still looking for his Maid Marian, but the Doctor remains distrustful of Robin and is convinced there is something wrong, particularly with the weather – it is far too green and sunny for the season.
Robin, his men, and the Doctor and Clara attend to an archery contest held by the Sheriff of Nottingham. Robin, disguised as Tom the Tinker, faces off against the Sheriff in the final round to win the prize of a golden arrow. As legend states, Robin is able to best the Sheriff by splitting his arrow with his own shot. The Doctor then suddenly appears and challenges the result, readily splitting Robin’s arrow with his own. The two begin to challenge the other with more arrows until the Doctor impatiently explodes the target using his sonic screwdriver. The Sheriff, intrigued by the Doctor’s power, commands his knights to capture him. When Robin cuts off one of the knight’s arms, it is revealed they are disguised robots. The Doctor allows the robots to capture him, Robin and Clara so he can learn more about the Sheriff’s plans.
The Doctor and Robin, in trying to come up with an escape plan, bicker to a point where Clara’s admonishments make her seem their leader by those watching, and she is taken to meet the Sheriff herself. Clara is able to manipulate the Sheriff into revealing his plot: he witnessed a spaceship crash and has been trying to repair it by collecting all the gold in the nearby land, so that he can use it to travel to London and take over the kingdom and eventually the world. Meanwhile, the Doctor and Robin manage to escape their confinement and locate the spacecraft. The Doctor realises that the craft had fallen back through time from Earth’s future on the way to the Promised Land (as mentioned by the Half-Faced Man in “Deep Breath”). Disguising their ship as the castle, the robots took on the guise of knights to further the illusion. The Doctor determines that the engines were damaged in the crash and are leaking radiation into the atmosphere, causing the surrounding area to be more like the Sherwood Forest setting of the Robin Hood myth. He also discovers that the craft’s data banks contain the myths and legends of Earth’s history, including Robin Hood. The Doctor becomes convinced that Robin is a robot, created by the “knights” to give the people false hope. The Sheriff arrives with Clara in tow, having been alerted to the Doctor’s presence. Clara and Robin jump out of a window into a moat below and escape, but the Doctor is taken prisoner again.
Robin and Clara reunite with the Merry Men, where Robin, shaken by the Doctor’s revelations about his future, commands Clara to tell him who the Doctor is and what he knows about the legend of Robin Hood. Meanwhile, held captive in the gold refinery, the Doctor realises that the robots are creating a “gold matrix” to repair their ship’s engines. However, there is not enough gold in the area to fully complete the repairs on the spacecraft. If the Sheriff should try to use it, it will destroy “half the country”. With the assistance of a young woman held captive by the robots, he leads the prisoners in a revolt against the knight robots. Most of the robots are destroyed, and the prisoners flee. The Sheriff appears and while trying to persuade him to abandon his plan, the Doctor realises from the Sheriff’s reactions that his belief about Robin was wrong — he is the real Robin Hood. At this point, Robin, Clara and the Merry Men arrive to save the day. The Sheriff challenges Robin to a duel. Robin’s arm is wounded in the fight, but he defeats and kills the Sheriff by knocking him into one of the gold vats with a move that the Doctor used against Robin in their earlier duel.
The spaceship, controlled by the few remaining robots, takes off and tears apart the castle. The Doctor and Clara help the surviving prisoners to escape. Knowing that unless the ship achieves orbit, it can still decimate the countryside, the Doctor plans to provide just enough gold to the craft to give it the spurt of power needed to force it away from the Earth’s surface. Since Robin’s arm is injured (and the Doctor cheated during the archery contest), the Doctor, Clara, and Robin work together to fire the golden arrow from the contest into the ship, which is then able to climb above the atmosphere where it detonates harmlessly.
As the travellers prepare to leave, the Doctor admits to Robin that while he still cannot accept his story as real, he confirms that Robin will be remembered as a legend rather than as a man. Robin takes a moment to accept this, then states that the Doctor, based on the stories that Clara told him, is a hero of similar background as well. The Doctor denies this, but Robin Hood suggests that their role is not to be heroes themselves, but to inspire others to take on that role. The Doctor and Clara depart, the Doctor leaving Robin a gift – he found Maid Marian (revealed to be the woman who helped him lead the prisoner rebellion) and reunites her with Robin.