After waking up from the long winter sleep, the Mumintroll and his friends Snusmumrik and Sniff decide to do something exceptional to celebrate the first day of spring. During their morning expedition, they make an unusual discovery that ends up giving them not a few surprises and the occasional great scare ... To change the air, the whole family decides to take a boat trip to a supposedly deserted island, but not even so far from home. They manage to evade the spell they've fallen into, and the arrival of two new guests on the way back will only further complicate matters. It takes all the good judgment of the Snork to avoid greater evils. It will be an unforgettable summer for all the animals of Valle Mumin ...