Dr Lanyon's letter tells of how Lanyon received a letter from Jekyll asking for him to collect a drawer that contained some chemicals, a vial and a notebook from Jekyll's laboratory. Lanyon was to give it to a man who would call at midnight.
Lanyon does this and at midnight a man arrives who is small and grotesque, wearing clothes that are far too big for him. The visitor offers to take the chemicals away or drink the vial in front of him. Lanyon accepts this and watches as the man drinks the vial and transforms into Dr Jekyll. Lanyon is horrified and becomes very ill.
In the other document is Dr Jekyll's full statement of the case. Jekyll tells the story of how he turned into Hyde. He details how, what started as scientific curiosity of the dual nature of man and a desire to destroy his "darker self" became an addiction to the Hyde persona who increasingly took over until he destroyed Jekyll.