故事描述大魔头“黑暗”准备将这个世界的光亮吞噬，使人类永远生活在黑暗中。森林小子杰克跟他的十一位好友决定联手对抗黑魔王，方法是杀死各地的独角兽。最后，杰克还赢得莉莉公主的芳心，打破了黑魔王迎娶公主的美梦。
The Lord of Darkness (Tim Curry) alone in his chamber, calls his minion the goblin Blixx (Alice Playten). He confides to Blixx that he wishes to kill the last two unicorns so that darkness will fall upon the world, allowing him to emerge from his cave where he is shielded from the sunlight that is his enemy.
He cannot approach the unicorns, he tells Blixx, because only the pure can find them. Only someone innocent and pure, such as the Princess Lily (Mia Sara), can attract the unicorns. He sends Blixx and his two companions, Pox (Peter O'Farrell) and Blunder (Kiran Shah)to set a trap with orders to bring the unicorn's horn back to him.
The scene shifts to Princess Lilly all innocence and mischief, heading to the enchanted forest. Along the way the playful young teen visits her friend Nell (Tina Martin) a kindly peasant who lives at the edge the woods. She warns Lilly that visiting the woods is not a princess-like activity, but Lily goes anyway, leaving as quickly as she arrived amid Nell's warnings of things to forest things avoid. Lily's real plan is to visit her beloved Jack(Tom Cruise), a forest child and trusted friend to all the creatures that live there.
Unknown to the two lovers, Blixx and his companions have found them and although the goblins are repelled by the teens' beauty and goodness, they follow Jack and Lily knowning that the youngsters will sooner or later attract the targeted unicorns.
Unaware of the danger, Jack and Lily play in the forest glades together like children. Then Jack tells her that he has a special surprise, taking her to see the last two unicorns - a mare and a stallion. These are sacred animals, guradians of purity, innocence and all that is good. Mortals like Jack and Lily are not permitted to see--let along touch them, but Lily, delighted by their beauty and against Jack's warning--approaches them and gently pats one on the nose. Although the unicorns appear to be as pleased by Lily as she is by them, she has unwittingly made them vulnerable.
Blix, hiding from sight, seizes the opportunity presented by Lilly's charm of the unicorns, to fire a poisoned dart into the flank trusting stallion. Stung and in pain, the stallion gallops away followed by the mare. Blix's party gives chase while unknowing, Lilly, delighted by the experience, but puzzled by the unicorns sudden departure, returns to Jack. But Jack is very angry at her for desecrating the unicorn with her human touch.
Lily, though not at all contrite, apologises teasing Jack by telling him that she will marry the man that finds her ring. She tosses the ring and it falls into a pond. Jack leaps to his feet and dives into the water to retrieve it.
Meanwhile Blix and his party overtake the dying stallion and cut off the magical horn. Without the power of the stallion the world plunges into sudden, severe winter. The onset of a blizzard and the howling winds drive Lily to seek shelter. Meanwhile, below the surface of the pond Jack seeks unsuccessfully for Lily's ring. Out of breath, he swims to the surface but his way blocked by thick ice. Using the last of his strength to smash though, he crawls, gasping, to the shore, perplexed by the sudden snow and cold.
Lilly, meanwhile, seeks refuge at Nell's house, but finds everyone and everything inside frozen solid. Hearing approaching riders, she hides in the loft when Blix and his triumphant party arrive seeking food and loot. Listening from her hiding place she overhears them bragging about what they have done and realizes it was her sin that provided them the opportunity. As they leave to deliver to horn to Lord Darkness she vows to follow them and make everything right again.
