Richard goes to England and there is one Robin who's a mercenary under his rule, Richard is hurt and dies after two weeks, in which we get to know Eleanor of Aquitaine and to learn that Richard doesn't know much about England or its language. Some noblemen conspire to achieve that Richard designs, as he said sometimes, his nephew Arthur as king, but Eleanor makes him change his mind and Richard dies after passing the crown unto John, because England needs a clever king, nor a child king who can be played by the nobility.
In the meantime, Richard, who was prisoner, escapes with his adventure friends and tries to get to the Channel.
Then, John falls in love with Isabelle of Angoulême, who is to marry Hugues de Lusignan. John repudiates his wife Ingeborg of Denmark and kidnaps Isabelle just before her wedding. Hugues, vassal of England but inside French territory, complains to his "natural lord", Philippe II of France, and Philippe gets a casus belli on England. Before that, we saw a scene shared by Philippe and the English knight Godfrey speaking about peace and French possessions, and the Angevin threat to France.
While John is suffering loss after loss in France, he's raising taxes in England to solve his budget problems (besides, Richard ruined the kingdom's income). He introduces extraordinary taxation for the nobility, who is outraged. Arthur of Brittany becomes a good choice for King of England in their eyes. We get to see Arthur as a kind-hearted boy, with intentions of being brave and just. William Marshall is contacted by Godfrey, and both of them go to Arthur to propose him an offer: he is to become vassal of France in exchange of help in his quest for the English crown.
Where is Robin? Like in the film, in a similar way, he gets to Nottingham and suplants the lord's son, just like in the film. The difference is that the Sheriff has more protagonism, because he represents the threat of the king raising taxes and trying to screw up the nobility. Walter of Nottingham is approached by William Marshal, and Robin meets William, old friend of Walter. William tries to talk Walter into supporting Arthur, because he is a prestigious lord and if he does go with Arthur, many will follow. Walter says he's too old and poor now. Life is good, somehow, in Nottingham.
Until the tax farmers come to Nottingham and sack the castle, led by the Sheriff's intentions of avenging his pride and trying to get Marian for himself. Driven by hatred and greed, the Sheriff fights and kills Walter. Robin, Marian and his merry men were away, trying to get the food from the bishop's convoy, and when they come back and they see that there is nothing left for them in the ruined Nottingham, Marian convices Robin to embrace his new identity as a nobleman and travel to Brittany, to swear loyalty to Arthur.
Robin, by the way, would be more interesting if he had some kind of internal debate between acting like a nobleman and acting like a low-born idealistic mercenary.
I could go on, centering the story in two points: John's capture and murder of Arthur, which lead the barons to revolt, and the suqsequent Revolt of the Barons.
Robin's adventure would be inside this plot, and easily done, especially when it comes to the teenager Arthur's murder. Robin would be close to Arthur's ideas, while Godfrey, or John's friend Braose, try to kill Arthur.