Modern AU. When Merlin Emrys gets a summer job at Buckingham Palace, he doesn't expect to even meet King Arthur, let alone become involved in protecting him from a plot to overthrow the monarchy.

Outside the city walls, the tower stood black against the sky.

In the not so distant future, Merlin builds a tower. That's not suspicious at all.

In which Prince Arthur meets Merlin and all hell promptly breaks loose.

OKAY OBVIOUSLY I COULD NOT RESIST THIS:

Modern AU. Arthur and Morgana are Metropolitan police officers drawn into a strange case involving robbery of ancient artifacts. Merlin is a graduate student at Imperial College studying the healing plants of Ancient Britain under Gaius – or at least that's what he tells everyone. Soon they, along with Gwen, Lancelot and Uther, will be facing a battle for the future of the country. Can they defend the realm from those who would seek to destroy it?

One would think being a famous astronaut, the darling of the Royal Spacefaring Corps and the heir to the throne besides, would afford him certain privileges. But oh, no, of course not; because he was the best pilot (or so they told him, and who was he to argue?) he got tasked with the most dangerous operations in the most experimental spacecraft. Which would not normally be a problem – after all, it wasn’t a boast that Arthur could fly anything with a fusion engine – but clearly the new Excalibur craft had been cobbled together out of spare parts and welded into a whole with chewed-up Maynard’s Wine Gums, because he was only halfway to Epsilon Eridani when bits began to fly off the bloody thing and he had to ditch on the nearest planet before the entire cockpit fell apart around him...

Uther fears his son's power, and Merlin has helped Arthur flee to escape imprisonment. Now the two of them must fight to regain Arthur's rightful place in Camelot.

The first thing Merlin Emrys thought when he was born, was "Oh, hell no."