The general plot: the characters have to get information about, then infiltrate a ceremony of an evil religious sect, to save the intended sacrifice. (Hey, does this sound familiar? Ahhh, so what?)
If the party think they the goodies, then just drop them some rumours about the cult and its sacrificial doings. This should be sufficient to draw in properly-played PCs. If they are not inclined to do such a thing for nothing, then have them hired on to do so (a wealthy parent is concerned their newly-converted child is going to be a sacrifice, or the established local religion wants a stop put to this heathen bunch and their evil practices).
In any case, info about the cult should be sketchy. They'll have to sniff out rumours about town. They'll have to pay some snitches (who may or may not give them good information). They should get a little too close for someone's comfort, and they should get attacked for it. Maybe with magic, a curse or something serious.
Specifically, they'll want (i) cult background and (ii) location of the next sacrificial ceremony. The second will be very hard to get.
If some of the characters want to try to infiltrate the cult, this will be next to impossible. Unless some very convincing magical method is possible to replace an existing cultist, they'll be found out. If they try to "join", they'll be turned away and everyone's guard will be up.
If they manage to locate the ceremony, they'll witness terrible magic and nasty rites. An attempt to rescue sacrifices will be met with (i) much priestly magic (i) moderate armed resistance. If the party is strong, and they let the ceremony proceed at all before stepping in, they'll be confronted wit whatever terrible otherworldly thing the cult was attempting to summon with the sacrifice (its notice was gained with the beginning of the ceremony, and it is furious at the sacrifices which it did not receive: it'll appear and attack everyone, cultists included).
There's also a chance that they'll get captured themselves and get added to the list of sacrifices. This is a good way to go if the party splits up and comes in from two different directions, or tries to co-ordinate two different tasks at once: one group will suddenly notice that the other has been captured and is being led to the slaughter. This adds urgency to their task. 8-)
If this is the case, a good hook is to have the cultists do something to one of the characters, pre-sacrifice, that is somehow permanent: a terrible scar, or a spell, or some such. Something that they have to live with (and role-play with) for the rest of their lives, preferably.
There's lots of good hooks from the after effects, if they're successful in saving the sacrifice, or even killing off most of the cultists. They'll earn the undying enmity of that sect, and they'll be after them forever. They may get some nasty curses from the priests before they win. They may have allowed a terrible monstrosity to be summoned, which will wreak havoc, of course. They may even get blamed for this. Perhaps they'll discover that local officials or nobles were somehow illegally involved (they belong to the cult, or have struck a black deal with them). Will they reveal such a conspiracy? Can they really prove it?