Mr. Utterson, a lawyer, is a friend of Dr. Jekyll's.  Some time previously, he arranged Dr. Jekyll's last will and testament.  It included a strange condition:

in case of the decease of Henry Jekyll, M.D...all his possessions were to pass into the hands of his "friend and benefactor Edward Hyde," but that in case of Dr. Jekyll's "disappearance or unexplained absence for any period exceeding three calendar months," the said Edward Hyde should step
into the said Henry Jekyll's shoes without further delay and free from any burthen or obligation beyond the payment of a few small sums to the members of the doctor's household.

When the will was made, it bothered Utterson that Dr. Jekyll would give all his money to an unknown person.  Now, Utterson is even more troubled because he has recently found out who Mr. Hyde is.  Hyde is the hideous-looking man who knocked over a little girl on a street corner in the middle of the night, and then proceeded to enter a mysterious-looking apartment and come out with a check for 100 pounds--drawn from someone else's account!

Mr. Utterson is confused and troubled, so he goes to speak to Dr. Lanyon, who is an old and dear friend of Dr. Jekyll.