Final answer:
Friar John was unable to deliver Friar Laurence's letter to Romeo due to being quarantined because of a suspected plague outbreak, leading to a critical misunderstanding and the play's tragic climax.
Step-by-step explanation:
In Act 5 of Shakespeare's Romeo and Juliet, Friar John is prevented from delivering Friar Laurence's letter to Romeo because he gets quarantined. A suspected outbreak of the plague means that Friar John is unable to travel to Mantua to deliver the message which explains Juliet's false death. This misunderstanding is a pivotal moment in the play, leading to the tragic climax where Romeo, believing Juliet to be dead, takes his own life.