Boogeymen (Star Trek: The Next Generation, No. 17)

Eric Baldwin is the Federation's premier exologist, a specialist in all manner of alien life forms -- and one of Captain Picard's oldest, most trusted friends. But Baldwin's discoveries have made him enemies across the galaxy, and now he wants Picard to help him by erasing all traces of his existence.

but Picard soon finds himself with little time to worry about Baldwin's problems. For the U.S.S. Enterprise™. has suddenly become a strange and dangerous place -- a ship where assassins lurk in every corner, and even old trusted friends are not what they seem. Threats all masterminded by the strangest race of aliens Picard and his crew have yet encountered...