This plot is intended to be the backbone of a long campaign lasting perhaps years and requires a lot of continuity. If your players are the type that gets their characters killed fast and often, then this one is not for you.
Requirements: At least one high-level fighter, paladin or bard who always acts like a hero and does heroic deeds (called the hero later on). The party should use some specific Inn or hotel as their base between adventures.
During their first or second visit in the Inn the party meets a lovely, black-haired, tanned young lady who belly-dances for the audience. The girl should be exceptionally beautiful and charming, so that the characters are bound to notice her.
After the show the girl dresses up and comes to sit by the same table with the player characters. She introduces herself as Alanil (use whatever name you like) and tells that she is with another adventuring party. It should become apparent that she is somehow interested in the heroic character (choose a handsome PC that can't resist women), but when the hero tries to make his move, the girl says that she is already involved with someone else. This someone is Stompur (again, change the names if you wish), another great hero known for his big muscles and bad temper. Make it clear that Stompur is someone you wouldn't want to make angry.
Every time the party visits the Inn the hero meets Alanil and gets a chance to spend some time and talk with her. Slowly the hero gets the impression that Alanil cares for him and the feeling should be mutual. At the same time between Stompur and the hero develop a hatred for each other. Anyway, nothing more serious ever gets to happen. Stompur is out for an adventure most of the time and his reputation grows steadily - so does the hero's reputation.
After a year or so Alanil tells the hero that she senses something bad, but does not know exactly what it is. Soon after this the party will meet another gorgeous young woman, Shenadal, who wants to join them. Shenadal is as beautiful as Alanil, but blonde. Little by little the players should learn that there is something very similar about these two women, but they can't figure out what. Shenadal also likes to spend a lot of time close to the hero. The two ladies never meet.
Time passes and Shenadal adventures with the party. The heroes' reputation keeps on growing. One day the two parties (the players and Alanil's party) meet accidentally. Something very strange happens, the girls begin to change their shape...
Both Alanil and Shenadal are powerful ancient dragons who have polymorphed themselves into young pretty girls, so that they can easily get close to heroic fighters. The dragons have had a competition going on for centuries: the one that eats more and greater heroes in 500 years wins!
This plot gives you a chance to surprise you players in a way they've never experienced. Someone they've known for a long time wants to eat them up! They will have to fight side by side together with Stompur, whom they hate, against the dragons, whom they have learned to like or even love.