The Secrets of the Hattifatteners is the fifth episode of the 1990 television series Moomin.
The episode opens where the previous one left off. Moomin and his family are sitting in a tent on the Hattifatteners' island since the weather is too stormy for them to sail home. Moomin notices one of the tent's ropes is loosening, so Moominpappa tells him to have a look before the wind blows the tent away. Moomin obeys and ties the rope really tight to the tree.
Meanwhile, the Hattifatteners are still charged up from the lightning.
Moomin looks at the barometer, which will tell them what the weather is like. Snufkin believes that the storm will stop soon. Moominpappa tells Mr. Hemulen to wake up and tells him to return the barometer to the Hattifatteners. Mr. Hemulen objects by telling him that he will keep it so he can be reminded of the island. Snufkin reminds him that it will also remind him of the Hattifatteners, as well as thinking that the plants he found on the island is going to be enough. Mr. Hemulen says he only found 1 special flower and he nearly missed it. It's a small black flower, which he discovered as he was about to tie up the boat. Mr. Hemulen remembers that he may not have tied up the boat at Moominpappa's request.
Little My complains that they could be stuck on the island forever if the boat's gone. Moomin decides to go and have a look, as Mr. Hemulen begins to remember what he was doing. Snufkin follows Moomin, who finds the boat. The pair successfully bring it ashore in the nick of time. Snufkin tells Moomin to tie the boat to the tree and make sure that it's tight, which Moomin does. Snufkin thinks that the waves must have been huge to reach the boat and Moomin says that the storm will be over soon.
When they return, Snufkin tells everyone that everything's fine, much to Little My's relief. As Mr. Hemulen apologizes for not tying the boat up, Moomin sits down next to Snorkmaiden. Snufkin decides to play his harmonica.
As the Hattifatteners travel across the island, Mr. Hemulen tells everyone that the storm will be over soon as it's stopped raining. Moominpappa agrees since the wind has stopped. Snorkmaiden is relieved because the thunder and lightning was very noisy. Mr. Hemulen decides to go for a walk before he goes to sleep. Snufkin tells him that it's too dark to see, but Mr. Hemulen replies that the fresh air will do him good. Little My and Snorkmaiden decide to follow him.
Mr. Hemulen suddenly runs back inside the tent. Snufkin looks outside and sees hundreds of glowing Hattifatteners, which he believes were recharged by the lightning. Moomin wonders what they want. Moominmamma thinks that they're fairly friendly and want to see what they look like. Snufkin thinks that they want their barometer. Moominpappa agrees and tells Mr. Hemulen to give the barometer back to them, but Mr. Hemulen is too scared, so Moomin decides to do it instead. Moominmamma tells him to be careful and Snorkmaiden says that Moomin is very brave.
Anxiously, Moomin puts the barometer down on the ground. Suddenly, the Hattifatteners surge their way towards Moomin and electrocute him on contact. Snorkmaiden rushes over to save him. Luckily, Moomin manages to get out of the crowd of Hattifatteners, who make off with the barometer. As Snorkmaiden checks on Moomin, he tells her that he's glad his family don't use electricity.
A Hattifattener zaps Snorkmaiden's hair and Moomin wards it off. Snorkmaiden thinks that it was only a minor shock, until she smells something funny. Moomin points out her hair, and she runs over to Moominmamma, asking for a mirror. After a little searching her handbag, Moominmamma gets one out. Snorkmaiden uses it to look at her hair and is horrified when she sees it burned. Moominmamma tells her that they'll put some magic ointment on it, which will cause it to come back lovely and curly. Snorkmaiden replies that she doesn't want curly hair, she wants her long beautiful hair, before crying into Moominmamma's shoulder. Moomin tells her that her hair doesn't matter to him. Despite this, Snorkmaiden continues crying. Mr. Hemulen is relieved that the Hattifatteners are gone, until he sees Snorkmaiden crying.
Later that night, Snorkmaiden is sleeping with flowers on her head.
The next morning, Snufkin walks up a hill and sees the Hattifatteners sailing across the ocean. Moomin is woken by Snufkin so he can see the Hattifatteners leave the island. Snufkin believes that they're heading to a secret island where they can live on their own. Moomin thinks that they're weird, to which Snufkin replies "it depends on how you look at them, Moomin. They probably think we're a bit strange too."
The family is having tea now. Little My chastises Snufkin for not waking her up to see the Hattifatteners. Snufkin didn't want to wake her up as she was fast asleep "snoring her head off". Little My angrily replies that it was Mr. Hemulen who was snoring. Both Mr. Hemulen and Snorkmaiden (who is wearing a headscarf) hope that they never see the Hattifatteners again. Moomin asks his mother if he and his friends go exploring, to which she and Moominpappa agree, since it's a nice day and there'll be lots of interesting things that the storm washed up. Mr. Hemulen decides to join them so he can discover some more rare plants.