On California's Channel Islands, a stuck vehicle in Arlington Canyon led to a discovery that upended decades of settled archaeology. Digging to free the truck, researchers found human remains buried 37 feet down, later dated to roughly 13,000 years ago, sitting in an undisturbed geological sequence that let scientists read the site like a layer cake. The find, known to archaeologists as Arlington Springs Man, became a direct challenge to the long-dominant Clovis First model, which held that the earliest Americans arrived on foot across a Bering land bridge. The film follows the archaeologists and geologists who argue instead for a Kelp Highway: skilled maritime peoples paddling and foraging along a coastline rich with kelp forests, moving into the Americas by boat well before Clovis hunters existed. Interviews and site footage walk through the dating methods, the marine geology, and the pushback from Clovis-first holdouts, building a picture of the first Americans as seafarers rather than plains hunters.