When her mother is forced to take a job at the hardware store due to lack of any graphic design jobs, and David screws up his audition for the Warblers due to bad luck, Hilda decides to help them both with a spell she finds in a library book. However, the spell has unforeseen consequences.
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The episode opens with a flashback of a year ago, when David tried to audition for the Warblers, but it ended in a disaster (he tripped, knocked over a plant, and tore down the stage curtains). Now, a year later, he tries again along with Hilda. Hilda is surprised to learn that David can actually sing, and cheers him on. Sadly, the audition goes wrong again when a bug flies into David’s mouth just as he wants to start singing, sending the boy into a coughing fit. Afterwards, Hilda and Frida remind David he still has 3 more days to audition.
Back home, Hilda finds Twig all alone since Johanna had to take a job at the hardware store by a lack of any design jobs. Hilda and Twig visit her at the store, but Johanna has no time for them since she has to help the customers. So they go to the library. There, Twig discovers a hidden room behind a bookcase, filled with spell books. Hilda picks one about helping friends, and discovers a ritual for summoning Tide Mice; magical mice that can help anyone achieve success. She decides to use this on David. Since the Librarian won’t let her take any of the spell books home, Hilda copies the page she needs.
Following the instructions, Hilda successfully summons a tide mouse for David, and gives it to him without his knowledge (Tide Mice are invisible to everybody except the enchanter that summoned them). Later that day, David auditions again, and this time passes the audition with flying colors. Now that she knows the spell works, Hilda repeats the process for her mom so she gets a graphic design job again and can leave her job at the hardware store. Again, it works; Johanna is hired by the Trolberg Bellmakers Corporation for their new advertising campaign.
As the days go by, both Johanna and David enjoy their new success. But Frida begins to realize something is not right when, in school, David sometimes enters a trance where his eyes glow, on other times he starts to roar, and once purple steam began to rise from his head. She shares her fears with Hilda. Alfur recognizes the symptoms as those of an enchantment, and Hilda admits what she has done. She brings Frida and Alfur to the library and shows them the book. When Alfur reads it, the three friends discover to their horror that the Tide Mice will steal the souls of those they have been given to after a 30 day trial period. Fortunately, the book also contains instructions for a counter ritual to banish the Tide Mice again, but since the 30 days are almost up it has to be done during the Warbler concert the following evening.
The following evening all Sparrow Scouts and their parents gather at the lodge for the performance. David easily steals the show, while Johanna also begins to suffer the side effects of the spell. At moon-rise, Hilda uses the counter spell and the Tide Mice leave their hosts. They’re now visible to everybody and cause panic among the audience. Since the ritual has to be finished within an hour, Frida and Hilda quickly capture the mice, and drag Johanna and David outside. As Johanna drives them to the tide pool where Hilda summoned the mouse, Hilda explains everything. Johanna enters a trance again, and the car drives off the road. Hilda is able to bring the car to a stop, next to the tide pool, where she puts the mice back in the water and instructs David and Johanna to say the final spell.
With the danger over, Hilda apologizes for what happened. As they leave the beach, nobody notices the tide mice climb onto Hilda.