A Harry Potter story in which a potion is important to the plot.
Heart Over Mind (Harry Potter) by Regann
Something odd about Hermione causes her to have unexpected reaction to a love potion. Only it's one which no one expected. How could a lack of reaction cause so much trouble? AU to Books 5 and 6.
Felix Infelicis (Harry Potter) by Juniperus
It is said that angels are bright still, though the brightest fell, but we know what the road to Hell is paved with...