Disaster strikes the coral reef when Rainbow Fish accidentally drops his final, precious silver scale. He watches in dismay as it sinks down into the dark, uncharted depths of the sea. His friends beg him to stay in the shallows, warning him about the creepy, strange creatures that lurk in the pitch-black water. But Rainbow Fish is determined to retrieve his scale. Summoning all his courage, he dives past the safe, sunlit waters and into the shadowy unknown. Down in the dark, he is amazed to encounter a glowing firefly squid and a host of other fascinating, bioluminescent animals. He quickly realizes that the deep sea is not a scary place at all, but a spectacular world filled with wonderful new friends who just happen to look a little different.