Mr. Hemul tells a mysterious story: somewhere beyond the lonely mountains lies a lake where a legendary fish is said to live—at least, that's what his great-grandmother told him. Moomin and his friends want to find out the truth. But it's not quite that simple, because his great-grandmother also said that the fish only jumps out of the water after a violent thunderstorm. Moomin and his friends set off as dark clouds darken the sky. They have just reached the eerie Witch's Forest when the thunderstorm breaks out.