On a typical late summer day a baby sea otter washes up on the beach in Monterey, California -- hungry, lost, injured. It's a tragic event, but not surprising. California sea otters are struggling. For decades marine biologist Karl Mayer and his small staff have worked unceasingly -- one otter at a time -- to bring this "keystone" species back from the brink of extinction so it can play its important role in the local marine environment. But the effort has stalled, and no one knows why. This is the story of the Monterey Bay Aquarium's 501st attempt to save an orphan otter. From her discovery as a stranded newborn pup crying on the beach through her rehabilitation in secret roof tanks atop the Aquarium, we follow as Otter 501 learns how to dive, hunt, eat, and fend for herself in the wild, where survival is a long shot at best.