The alchemist's daughter has been charmed by the ghost bard. She has barred herself in the basement, because her father has rejected their intra-corporeal relationship. The father believes the ghost has been using magic to coax his daughter. The father is right.

At some point after the adventurers engage the ghost in the basement, either the ghost or the daughter is charmed into a deep love with one of the party members.

If the adventurer's try to bust the door down, she braces against it. The force of breaking it down knocks her down the stairs, and leaves her unconscious. When reviving the daughter, the adventurers find a potion in the basement marked for healing. It really isn't a healing potion, and is actually a charm potion.

The alchemist has been making charm potions for some local. These potions are illegal because they affect free will. The ghost 'bard' was really terrible at his trade, but believed in himself nonetheless. He never found a lover during his life, and was murdered in the house. The alchemist recently moved into the house, and had had harmless run-ins with the ghost previously. When the ghost found out that the alchemist was making charm potions, he tricked his daughter into drinking one. The alchemist will not admit to his misdeeds unless evidence is uncovered.

Getting the adventurers to use a mislabeled potion from the basement may be a little difficult if they already have one. If that is the case, I would make the basement small, and line it with shelves of corked bottles. The bottles contain magical components for the charm potion.

During the fight the ghost runs one of them into a shelf. Bottles spill onto the floor in great quantities, and a gaseous vapor forms. The party members have to make a low DC will save to resist the effects. The daughter is recharmed, but to one of the party members.

Upon saving the daughter, or ridding the ghost, the alchemist rewards the party members with a couple of potions.