Picture the ultimate World War I veteran: 18 months in the trenches, 17 battles, two Purple Hearts, and the single-handed capture of a German spy. Now picture that this decorated combat hero didn't wear boots. He had paws. He was a brindle stray named Sergeant Stubby.

This episode tells the gritty, profound story of how the ancient instincts of a stray dog became the perfect countermeasure to the horrors of modern warfare. Beyond the novelty, it's a look at survival in the darkest conditions and the extraordinary bond forged between humans and animals when everything else is falling apart.

• Found wandering Yale's campus in 1917, Stubby was smuggled to France by Corporal James Robert Conroy and won over a furious commanding officer with a trained salute
• With up to 300 million olfactory receptors, he smelled mustard gas long ...