Fan fic sux because it's a mecca of crappy stories built on top of crappier ideas.
Here's a mocking example - the story revolves around a time traveling treasure hunter, we'll call him Colorado Jack. He goes back to find the sword of King Arthur and ends up teaming up with Merlin against Natzis that show up to get the sword for themselves. After some classic Indiana Jo-er-Colordo Jack witty banter, Saruman shows up and attacks Merlin. Battle ensues. Luckily Colorado Jack has his trusty Time Gate and calls in reinforcements. The reinforcements stream through the portal with their trademark battle cry of "Yo Joe!". The Nazi's call in their own reinforcements. A Delorian shows up with Michael J. Fox's head mounted on the hood and out steps Darth Vader. Suddenly, comets streak through the sky and crash near the battlefield. But they're not comets, they're space ships. And out step Optimus Prime and Megatron! Cue The Touch by Stan Bush. Who will win this epic fight?
Actually...that's pretty bad ass.
