One of those "fixed point in time" moments

Amy Pond turned her nose up as she trudged after the Doctor, taking care to avoid the muddy puddles and piles of animal dung.

"What are we doing here, Doctor?" she asked. When she'd asked to see all of time and space, she hadn't meant some middle ages English town.

"Coventry, 11th century," the timelord muttered. "I wasn't intending to bring us here, but the TARDIS detected an anomaly, and re-directed us."

"Well, that'd be the first interesting thing to happen in Coventry," Amy joked.

"Don't you know your history, Pond? This is the day of Lady Godiva's famous ride through the city. At least, it should be..."

Amy had long given up on trying to follow the Doctor's verbal stream of rambling, so did her best to keep up with him while trying to avoid dirtying her shoes. The Scot knew that her 21st century clothes were anachronistic, but she didn't plan on being here very long, and wouldn't be seen dead in the peasant rags the locals were wearing!

The ginger girl almost walked straight into the back of the timelord when he stopped dead.

"It should have happened by now," the Doctor explained.

"Good for her, I guess?" Any shrugged. "She doesn't have to ride starkers on a horse."

"No, Amy, you don't understand," the Doctor said, grabbing her by the shoulders. "This is a fixed point in time. If Lady Godiva doesn't complete her ride, her husband doesn't lower taxes, life gets progressively worse for the people of the city, uprisings happen...so many lives will be lost..."

His eyes twinkled as the outline of an idea began to form in his eyes. Standing back, he looked his companion up and down.

"How good are you at riding a horse?"