Moving to the end of the play then the plan the friar concocts is designed to solve the dilemma that Romeo and Juliet find themselves in. Drinking one of his potions will make Juliet for all intents and purposes appear dead. The friar's messenger will get to Romeo before he hears the news, and the friar and Romeo will revive Juliet and Romeo and Juliet can run off together and live happily ever after.
However, although Juliet drinks the potion and everything proceeds as normal, Friar John, the bearer of the message, never makes it to Mantua to deliver the message, and is confined in a quarantined house.
So, Romeo hears of Juliet's death and decides to kill himself next to Juliet. He buys a vial of poison from the Apothecary, and goes to the Capulet tomb in Verona. There he meets Paris who is scattering flowers on the grave of Juliet. They fight and Romeo kills Paris before drinking the poison and dying next to Juliet. Then Friar Lawrence arrives and Juliet wakes up. Friar Lawrence realises the massive disaster that has occurred and asks Juliet to flee with him to spend her days in a nunnery. Then they hear the town watch and the Prince coming so Friar Lawrence flees, Juliet tries to kill herself by kissing the lips of Romeo, then plunges a dagger in her body.
When you think about it, Friar Lawrence was quite arrogant in assuming his plan would work - it depended on a number of factors. But then, fate as a power in the play will not be challenged.