Tristran (Charlie Cox) is a young man hopelessly in love with a local village girl named Victoria (Sienna Miller). But while he nightly professes his affection for her, she only has eyes for what she considers a more suitable match, the rich and socially acceptable Humphrey (Henry Cavill). Still Tristran isn't about to give up on his quest to win Victoria's heart.
One night, after inviting her out to share a glass of champagne in the forest with him, Tristran sees a falling star and vows to bring it back to her as a sign of his undying love. More than willing to get rid of the pesky shop boy for a while, Victoria gladly sends him on the quest.
However, when Tristran arrives at the point of impact, he finds not a cold, hard lump of space rock but a beautiful, humanlike girl who's been injured during her heavenly fall. Still determined to complete his mission, Tristran starts the long walk home with Yvaine (Claire Danes) in tow. Nevertheless, he's not the only one interested in her arrival.
Three, moldering, decrepit witches have been anticipating her fall for centuries. By cutting out Yvaine's heart, the witches' youthful beauty and vigor will be restored. Leaving her two sisters (Sarah Alexander, Joanna Scanlan) at home to prepare for their transformation, Lamia (Michelle Pfeiffer) sets out to capture and dismember the celestial body.
Meanwhile, the king of Stormhold (Peter O'Toole) has died, leaving his three living sons (Mark Strong, Jason Flemyng, Mark Heap)--and the four dead ones in the form of lively ghosts (Rupert Everett, David Walliams, Julian Rhind-Tutt, Adam Buxton)--to fight for control of the kingdom. Eager to find the star for themselves and thus gain everlasting life, the rowdy brothers also search for the twinkling being as she holds a stone that will divine the next royal.
Yet witches and would-be kings aren't the only trouble Tristran and Yvaine encounter. Shanghaied by a shipload of air-borne pirates and their fierce Captain Shakespeare (Robert De Niro), the hapless couple is threatened by the raucous band of buccaneers and nearly undone by a shady, black market merchant (Ricky Gervais) who wants to sell the luminary and make a quick buck.
It all makes for a wild, swashbuckling adventure with plenty of swordplay and numerous depictions of impaling and death. Added to the physical blows are magical spells and evil sorcery meant to hamper the humans as well. Yet despite the frequent and sometimes graphic portrayals of murder (along with a pair of implied, casual sexual encounters, a cross-dressing bandit and a few moments of bawdy humor), the film unfolds as a quirky comedy reminiscent of Monty Python and the Holy Grail.