Hrm. A toughie.
As the PCs feel disoriented from the planar jump back to the material plane, they notice an empty chest in one of the PCs apartments. (A search DC of 20 will reveal a potion of water breathing) A note is inside it that reads:
I was here before you
I took the key
A family heirloom stolen
Come get it from me ...
One of the PCs kept his family heirloom in there, locked. It was passed down from generations. It was said that it helped destroy an undead menace ages ago.
After reading the Note (a scroll) the PCs are teleported to the plane of water where a showdown must occure between the Undead Mind Flayer menace and them