Darby helps to rescue a wild mustang she names Hoku; but her relationship with Hoku is cut short when she must set her loose among other horses and her original abusive owner decides he wants her back.
Ellen Carter, who grew up on Wild Horse Island, returns there to bring her daughter Darby back to California, and when Darby finds a wild mare in the forest, she tries to convince her mother to stay.
Darby and her friend Cade, trying to locate Cade's mom Dee who has been missing since a tsunami swept the island, hope Dee's pony Honi can lead them to her.
When Black Lava and his herd are sent up Sky Mountain to keep them away from polluted water, they encounter Snowfire, a white stallion that has claimed the mountain for itself, and Darby tries to intervene with the battle before Black Lava gets into trouble.
Adjusting to her new life in Hawaii, Darby Carter is heartbroken when her new colt, which she found abandoned on a beach, turns out to be owned by her new enemy, her snobby cousin from across the island.
After the filly she saved in Nevada arrives at her grandfather's Hawaii ranch, Darby realizes the horse is in bad condition and she must try and save it.
Uncertain of her horsemanship, Darby starts to see a mysterious horse prowling the grounds of her grandfather's ranch at night and begins to wonder if it is the legendary Shining Stallion of Hawaii.
Darby visits her great-grandmother, a healer who lives in the rain forest, connects with a wild mustang, and tries to mend the relationship between her two best friends, which was broken by a secret tragedy.
