Darby is excited to go on a "volcano watch," camping out and observing the island's volcanoes, despite recent small earthquakes that could mean an eruption is coming. After all, Darby grew up in California, so she's used to the earth rumbling now and then. Plus, the expedition is perfect for working with Hoku, helping the young mustang learn to handle any situation. But when the volcanoes do erupt, it's much scarier than Darby anticipated, and a stream of lava leaves Darby and Hoku stranded. Can Darby's horse-charming skills keep the mustang calm? Or will her filly run back to the wild?