Emmett and Verne Brown arrive from 2017 in Timeline 18. Emmett gives Clara the Jules Verne novel The Lighthouse at the End of the World, which Verne will not write for another 12 years, in celebration of their anniversary. Doc then opens Clara's gift, a notepad and fountain pen, which will allow him to take notes and not forget important dates.
Doc arrives at his secret lab and discovers a letter that Marty had written on April 15, 1986. Doc then departs for that date.This creates Timeline 20, which begins at Doc's entry point in 1986 and timeline 19 fades away.