Rewrite this grammatically troubling movie poster

On a Wiki-walk the other day, I stumbled across the movie poster for Devil. The tagline on the poster is:

Five strangers trapped. One of them is not what they seem.

The writer was obviously using a singular "they" to avoid specifying the gender of the Mole, so as not to give away part of the plot. Sometimes singular "they" works for me, sometimes it doesn't. In this case, it really struck me as jarring, but I can't come up with a better alternative.

Can you?

Solution 1:

At the cost of changing the meaning, we can avoid saying something awkward:

Five strangers trapped. Not all of them are what they seem.

Would this change make the film seem less or more exciting? :-)

Solution 2:

Five strangers trapped, and four of them are just what they seem.