One of the oldest, most successful arthropods on Earth, horseshoe crabs have existed for over 350 million years. But in recent years their populations, have crashed by 75% from overfishing, resulting in moratoriums on catching them. Few of us know that most human lives depend on the valuable, shockingly blue blood of these "living fossils." Even more surprising, a tiny shore bird, the red knot, is so dependent on them that the crabs' population crash may make these long-distance flyers extinct. This mystery of mutual dependencies emerges from filmmakers and scientists revealing a disturbing ecology story ranging from the Arctic regions to the southern tip of the Americas.