The Secret of the Hattifatteners is the sixth episode in the first series of the 2019 television series Moominvalley.
Moominpappa recalls his adventures sailing with the Hattifatteners. He speaks of how he spent many months with them, wanting to disprove the "nasty rumours" that they led a wicked life, and he became quiet and mysterious, like the Hattifatteners. One day their boats arrived on an island, each boat arriving with an odd number of Hattifatteners, with all of them drawn towards some unseen force. Moominpappa is about to reveal what drew the Hattifatteners to the island, when Moominmamma arrives with marizipan pigs, and Sniff begins loudly eating them. Moomintroll and Snorkmaiden clutch onto each other and anxiously await the next part of Moominpappa's story, however he says that he cannot begin with the climax of the story, and starts from the beginning, to their disappointment. Later, Moomintroll and Snorkmaiden are playing at being pirates on a boat in a shallow part of the sea, near the beach. Though Moomintroll attempts to compliment Snorkmaiden, she continues to make him walk the plank, brandishing a wooden sword. Their homemade plank breaks and he falls into the ocean, the two of them laughing as he pretends to be attacked by a "killer shark" - a tiny fish. Moomintroll wonders if they are getting "a little old" for play-acting, asking Snorkmaiden if she would like to go on a real adventure. Snorkmaiden realises that Moomintroll may want to follow in his father's footsteps and says that it would be nice to "have a story to tell our children". Moomintroll becomes uncomfortable and suggests that they don't have to travel immediately though Snorkmaiden continues to insist, realising that it could be an opportunity to find out what happened at the end of Moominpappa's story. While Moomintroll continues to backtrack, saying that it will only be "a few months" until they find out, Little My, who has hidden herself in a picnic basket on board the boat, un-fastens the boat from where it has been moored to the jetty, and the boat begins to sail out into the ocean. Snorkmaiden spots what seems to be a Hattifattener boat in the distance and Moomintroll begins to agree to the idea of sailing "a little way" out. Snorkmaiden then realises that the boat is already drifting out to sea and praises Moomintroll, thinking that he had already noticed the boat and cast them off - which he goes along with. Moomintroll brandishes a wooden sword and proclaims "follow that ship, me hearties" as Snorkmaiden hoists the sail. Little My, still unnoticed, eats one of the sandwiches in the picnic basket.
Snorkmaiden continues to steer the ship as they play-act, Moomintroll acting as captain and giving directions. They sail closer to the Hattifattener ship - which is revealed to look more like driftwood and is empty of any creatures. Snorkmaiden complains that "they tricked us", then notices the sound of thunder hailing an oncoming storm. Moomintroll suggests sailing out to an island in the distance for shelter, which they do, with Snorkmaiden a little unsure. When they arrive on land, Snorkmaiden says that she "can't imagine" why anyone else would come to the island, but Moomintroll remains optimistic, calling it "paradise". He suggests that Snorkmaiden finds them a cave while he carries the picnic basket, and she walks off into the distance. Moomintroll follows after her with the picnic basket, looking a little unsure but assuring himself that coming to the island was "much safer" than going home. In the distance, a fleet of small boats arrive.
Moomintroll finds Snorkmaiden picking a patch of flowers by a cave and crafting them into a crown on her head. Moomintroll comments that the picnic basket is "a lot heavier" than he remembers, and when Snorkmaiden suggests that they "eat what's inside" to make it lighter, they open the lid to reveal Little My, who says: "you're not eating me!" Snorkmaiden and Moomintroll are cross when they discover that Little My has been eating all of their food. Little My collects a torch from some abandoned equipment near the cave, and leaves to explore, calling Moomintroll and Snorkmaiden "lovebirds" before making a disgusted noise. Snorkmaiden tells herself that Little My is "jealous", because she is beautiful while the other "has a head like an onion". Snorkmaiden searches for a mirror in which to see her new look, with her wreath of flowers, and is disappointed when a spoon and a puddle fail to work. Moomintroll offers her a photo of herself that he keeps with him, but Snorkmaiden complains that is is the "old" her, and says that being shipwrecked isn't as fun as she'd thought. Moomintroll, on the other hand, says that he is beginning to enjoy it, and builds them a small shelter. Snorkmaiden asks if she can help, though Moomintroll points out that she can't, not now that he's finished, however he leads her away to search for something to eat. The storm grows closer.
Little My is elsewhere on the island, finding it "dull" as she's been unable to find anything she considers interesting. She hears electricity buzzing and shines her torch over the area, however her batteries run flat. A Hattifattener appears, which Little My doesn't recognise, and instead calls "a sock without a shoe". She traps the Hattifattener in her torch as a new battery, making the beam much brighter. The light lands on Moomintroll and Snorkmaiden, who Little My isn't happy to see. Moomintroll asks who else she expected to see, to which Little My replies "them" and Moomintroll and Snorkmaiden hold onto each other in fear before she shines her torch over a group of nearby Hattifatteners. Moomintroll explains what they are to Little My, saying that they "wander the world" and "don't seem to care about anything". He begins to repeat what his father had told him, and we are shown flashbacks of Moominpappa's adventures with the Hattifatteners. Moominpappa says that you can "never tell" what emotions a Hattifattener is feeling. Back in the present, Little My points out that the Hattifatteners are approaching them, and Snorkmaiden asks what they want. Moomintroll replies that "you can never tell". The Hattifatteners cause an electric shock, peturbing Snorkmaiden and causing her to run away, and then begin to crowd around Little My, who faces them angrily before Moomintroll appears and tells her to leave them alone. Her torch begins to shake while three Hattifatteners approach, buzzing with electricity. Moomintroll tells Little My to leave the torch and get away, however she tells him that she can't, and her and the torch are pulled into the group of Hattifatteners like a magnet. They move away at speed, taking Little My and the torch with them. Moomintroll is left alone, debating whether to go after Little My or Snorkmaiden, and he decides to search for Snorkmaiden as "then she can rescue Little My".
Snorkmaiden stops running and finds herself atop a mountain on the island, drawn towards something "sparkly" and "beautiful". It is her own reflection in a strange mirror, which has been hung on a single, wooden post atop the mountain. Snorkmaiden admires her reflection, and the flowers on her head. Two Hattifatteners approach, unsettling Snorkmaiden, until she realises that stamping her foot causes them to move a distance away from her. She decides to take her new mirror elsewhere, away from the Hattifatteners, and runs away.
Moomintroll arrives back at the cave, downtrodden after failing to find Snorkmaiden. He finds the photo of her and apologises to her picture for being unable to find her. When he lowers the portrait, however, he sees Snorkmaiden sitting outside of the cave, who says she is glad that Moomintroll is back, so that he can admire her "handiwork". Snorkmaiden gives her a tour of the cave, which has been decorated with lanterns and fishing equipment that she found nearby. Moomintroll says that it is "unbelivable", pleasing Snorkmaiden, until he explains that it's unbelievable that he was worriedly searching for Snorkmaiden when she had merely been at the cave the whole time. Snorkmaiden says that she can still tell everyone that he called her decorating unbelievable. Moomintroll, a little irritated, turns off a radio and inspects the mirror that Snorkmaiden has hung up on the wall of the cave. Snorkmaiden explains to him that she found it.
Suddenly, the fogged up glass of the "mirror" begins to clear, steam rising from tubes on it's sides, and a clock-like-face is revealed, with different weather symbols. As the clock hands land on a symbol, that type of weather happens, predicting thunder, then lightning, then rain. Snorkmaiden says that she is glad to be out of the rain, as it would make her hair become "spiky and nasty", and adds "talking of which, where's Little My?" Moomintroll looks unhappy, and Snorkmaiden correctly guesses that the Hattifatteners got to her. She tells Moomintroll that he "must set out alone" and risk "life and limb" to rescue her, and they seem to return to their play-acting, with Moomintroll gathering equipment and preparing himself while Snorkmaiden becomes impatient. Moomintroll dramatically bids her farewell and attempts to read a poem, and she pushes him out into the rain, right into Little My. Snorkmaiden reacts with surprise that Little My rescued herself, and Little My tells them that the Hattifatteners merely wanted the torch opened, so they could retrieve their "battery". Moomintroll is dumbstruck to realise that she used a Hattifattener as a battery.
A ticking noise from the weather-clock interrupts them before they can continue their conversation. Moomintroll and Snorkmaiden find the clock counting up to something, realising that it is counting the number of thunder-claps. As the thunder count reaches 300, the clock numbers roll over into images of Hattifatteners, like a slot machine. They return to the mouth of the cave, with Moomintroll saying that he thinks they may have "visitors", however they see nothing, until a flash of lightening reveals hundreds of Hattifatteners coming closer to the cave. Little My asks what they could want, as she already gave them their "battery", and Moomintroll says that he thinks he knows. As the Hattifatteners grow closer Snorkmaiden hides under a blanket and Little My disappears inside a teapot. Moomintroll tells Snorkmaiden that the Hattifatteners won't leave until they give them "what they want", urging Snorkmaiden to hand over the weather-clock, however she pretends she doesn't know what he is talking about. The electricity from the Hattifatteners switches on the radio and causes the lanterns to flicker as they enter the cave. One Hattifattener travels over Snorkmaiden's hair as she lies under a blanket on the floor of the cave, singing off all of her hair with a flash of electricity. When she asks Moomintroll what happened, he says it's "nothing to worry about", just a "new look". The Hattifatteners crowd underneath the weather-clock, proving Moomintroll's suspicions correct, and when he takes it from the wall they part to let him through. Snorkmaiden sees him leaving with the weather-clock, crying out "my mirror!" and Moomintroll reminds her that it doesn't belong to her. He tells her that the mirror surely isn't worth losing "anything else" for, and when Snorkmaiden asks what he means, he lets her take one last look at her reflection in the watch-clock, causing her to sob and hide under her blanket. Little My leaves her teapot and watches as Moomintroll follows the path the Hattifatteners have created.
Eventually, the Hattifatteners reach the top of the mountain, leading Moomintroll to the wooden post, where he replaces the weather-clock before scurrying away to peer out from behind a cluster of rocks. Little My is waiting there, also watching the Hattifatteners, and she seems confused about all of the trouble the Hattifatteners went to for a "clock". Moomintroll says that it isn't a clock, or a mirror, and he thinks it is "some kind of barometer", for predicting the weather. Little My suggests that the secret of the Hattifatteners is that they are "weather forecasters", though Moomintroll doesn't think so. They watch the Hattifatteners crowd around their barometer, bowing towards it in waves, while the storm continues overhead.
Moominpappa's voice narrates as the Hattifatteners bow towards the barometer, saying that "maybe the Hattifatteners gather together to greet the storm". A bolt of lightning hits the barometer, sending bright waves of electricity and then pulses of light throughout the crowd of Hattifatteners as they light up, forming shapes and patterns in different colours of light. As the Hattifatteners create this light display, Mooominpappa continues to talk, saying that perhaps they create the storm, or perhaps they'll "never know", and perhaps "that's the way it should be". The beams of light shine out in rings around the island and send beams into the sky in a dazzling display, which Moomintroll and Little My watch in awe. Moomintroll says that he supposes that's why they're always wandering, because they're looking for "the next great storm".
Moomintroll, Snorkmaiden and Little My sail back home once the storm has passed, watching the Hattifatteners boats sail off into the distance. Moomintroll says how lucky they were to witness the event, though Snorkmaiden, with her hair singed off, says that she wishes they'd never gone to the "silly island". Moomintroll finds her pirate hat and places it on her head, and he tells her "I'll be your mirror", saying that it looks much better. He attempts to brighten up Snorkmaiden, saying that "hair looks so untidy anyway", and she seems a little happier. Little My becomes hungry and reaches into the picnic basket for food, instead pulling out a lone Hattifattener, who electrocutes her before she shakes it off into the sea, where it swims off to join the Hattifattener flotilla. Little My's hair is stood on end, and she jumps into the picnic basket to hide when Snorkmaiden and Moomintroll giggle, Moomintroll commenting: "shocking".
The Secret of the Hattifatteners and Monster Fish are the only episodes to not feature or mention Snufkin.