PCs are dealing with some nomadic tribe (in my version, they were trying to set up a trade deal with them). Problem - chiefs brother has disappeared in mysterious circumstances (surprise me) and, guess what, the tribe is mourning and is not prone to doing business - so, if the intrepid PCs can rescue the brother, everyone'll be happy.
The brother has in fact tried to visit the ancestral plane to find out loads of Good Things, meet ancestors etc. He got the instructions from a ghost in an old ruined hill fort, which he got the location of from a diary he bought from some other nomads (the PCs can sort all this out with the right clues). He went to the hill fort, summoned the ghost, and got the spell to open a gate to the other plane. Unfortunately, the ghost being a miserable bugger, and the brother being of the trusting and slightly awed sort, the ghost withheld how to get back "for a laugh". So, the brother successfully built the gate, went to the ancestral plane, only to discover he couldn't get back. The PCs had better be more ruthless when talking to the ghost or exactly the same'll happen to them,
The way this scenario goes depends very much on what the PCs do - when I ran it, the main feat was getting to the hill fort and talking to the ghost - rounding up the components for the gate and rescuing the brother were fairly simple compared to that.