This is the story of two young girls. Flavia (Maria), is from a humanist family, who barely even bother with Christmas, and is just starting at her new school. Chance sits her next to Veronica (Rojo), an orphan who lives with her grandmother, who tells her tales of witches and other occult creatures, which have succeeded in convincing Veronica that she's a witch too. Flavia is initially sceptical, but it convinced after Veronica knows in advance of a teacher's departure, and then appears to make Flavia's hated piano teacher drop dead. Actually, Veronica just overheard the former, the latter had a previous health condition, but why let that get in the way of being able to turn your friend into a true believer, and make her do virtually whatever you want?
It's that angle of psychological domination that I find most interesting, and that's why the first half is a great deal stronger than the second, where it's replaced by the girls seeking ingredients for the titular potion. Frog-collecting, somehow, just isn't so captivating as watching the process by which one person call fall totally under another's... Well, spell, I guess. Taboada makes one striking stylistic decision, almost never showing adult faces in the entire film, which helps distance the girls from the adult world. I can't say the ending rang particularly true, with Flavia both finding a dubious line beyond which even she won't go for her friend, and reacting in what appears a rather implausible fashion [let's say, it demonstrates a greater actual savagery than Veronica]. Maybe they should have dumped the entire occult angle and just had the girls engaging in more prosaic mischief? That way, the steak might have ended up matching the sizzle. Sadly, that is not the case in the end product here, which does a much better job of set-up than execution.