The answer to the question :
How does Dr. Jekyll’s letter move the plot forward?
After reading the letter, Dr. Lanyon goes to Dr. Jekyll’s house and argues with him.
After reading the letter, Dr. Lanyon forms a plan with Mr. Utterson to help Dr. Jekyll.
After reading the letter, Dr. Lanyon reports Dr. Jekyll to the police.
After reading the letter, Dr. Lanyon breaks into Dr. Jekyll’s cabinet and takes his drawer.
Is:
After reading the letter, Dr. Lanyon breaks into Dr. Jekyll’s cabinet and takes his drawer.