"Bad Blood" is the tenth episode of Season 3 of Robin Hood and the 36th episode of the series as a whole.
Now that Gisborne has become an outlaw, Robin gleefully stalks his scraggly and underfed enemy through the wood. A mysterious hooded man shoots them both with poison darts, ties them up, and then when they awake, eventually reveals to them that he is Robin's long-lost, previously thought deceased father. The man also admits that he had an affair with Guy's mom and that Robin and Guy share a half brother and that the two must end their enmity for the sake of family, because apparently their half-brother is in jail in York, awaiting execution for an unmentioned offense. Flashbacks illustrate how Guy was once a well behaved, essentially good youth and Robin was a bratty, arrogant child who ruined Guy's life and sent Guy, and Isabella, into exile in France. This was never mentioned in any previous episodes by any of the cast. Also, apparently Guy accidentally started a housefire that consumed his parents (which is ironic as he intentionally burns down Marian's house when she betrays him). Despite the long history of violence and revenge between them, Robin and Guy decide to band togther to save their newly-discovered brother, Archer, understanding that they will also be working together to kill Guy's sister whom he's known about all his life and apparently Robin used to know as a child too.
- Robin Hood ... Jonas Armstrong
- Gisborne ... Richard Armitage
- Malcolm ... Dean Lennox Kelly
- Sir Roger ... Paul Hilton
- Ghislaine ... Sophie Winkleman
- Longthorn ... Ian Reddington