Here's an idea
This is the story that I used when I started a lower level Planescape campaign:
The individual party members are summoned by a powerful trader for a specific job to do. Once all of the members are presented, they are led to a meeting room and left to their own devices for a bit. They soon notice that a strange red gas has started to fill the room, and one by one, the PCs are knocked unconcious.
The game starts with the PCs waking up in chains, surrounded by a motley collection of mid to upper level demons and other creatures of the planes. They soon discover that they are awaiting trial in Sigil for conspiracy to upset the balance of Sigil itself. They also discover that they themselves look like an assortment of lower planar creatures. The judge addresses them by names they have never heard of, and passes sentencing on them for crimes they knew nothing about. They are sentenced to life imprisonment at the Vault, located on Carcera. The PCs are soon shuffled off to a holding cell where they await the opening of the gate to Carcera. There they meet a drow accolyte of Llolith, who says nothing to them, but will later play a huge part in the campaign. The group is then transported via a massive gate to the Vault.
A strange thing happens when they pass through the gate however. All of the PCs now revert back to their original forms. When the warden looks over the list of prisoners, he merely assumes it is a clerical error in the paperwork, and refuses to listen to the PCs pleas of innocence. The PCs are now wrongfully accused/framed prisoners on the plane of Carcera, and it's there goal to escape and get revenge on those that set them up!
I set the campaign in Carcera because of all the lower planes, it alone is probably the most survivable by lower level PCs, and it has a very clear cut goal for the PCs to strive for. By the time your PCs get off the plane of Carcera, they will be high enough level to tackle just about anything!
