Complex sentence: Marty gives Emmett a sealed note with instructions to be opened in the future, shortly after a second DeLorean appears with Doc at the wheel, having been summoned from the present by the note.
Split into simpler sentences: Marty gives Emmett a sealed note with instructions to be opened in the future. Shortly after, a second DeLorean appears with Doc at the wheel, having been summoned from the present by the note.