Cleaning urine off a mattress?

My beagle, who is typically house-trained decided she needed to go, right on my mattress. I didn't know until I was about to climb into bed. The stain had set for at least four hours. How do i get the smell out of the mattress? I don't care if it's a "natural" solution or some off-the-bottom-shelf cleaner so caustic I have to wear a haz-mat suit, I just need my mattress clean and smell gone! Help!