Star Wars Episode V: The Empire Strikes Back

The whole Luke - Leia - Han thing all seems normal in the first film but it gets rather messy in the excellent Empire Strikes Back. The tension is only resolved in the third film (third if you pretend the three prequels never happened) with the revelation that Luke and Leia are siblings - not that it really mattered as Luke was fairly a-sexual anyway. There's a brilliant Adam and Joe skit made with Star Wars action figures in which Luke seeks to spark up a relationship even with this knowledge.