Romeo and Juliet?FRIAR LAURENCE Hold, daughter: I do spy a kind of hope,Which craves as desperate an execution.As that is desperate which we...

Juliet is desperate to avoid being forced into polygamy by her father because Lord Capulet has ordered her to marry the County Paris, and she is already married to Romeo.  In her desperation, she threatens to kill herself.  Friar Laurence's monologue offers her "a kind of hope" to get her out of her dilemma.  He, too, is desperate to avoid the forced wedding because he is the one who married Romeo and Juliet behind their parents' backs.  Friar Laurence tells her that if she "hast the strength of will to slay" herself, then maybe she has the guts to "undertake a thing like death".  He is saying that if she is claiming to be strong enough to commit suicide to prevent the marriage, then perhaps she may be willing to go through with faking her own death.  If she agrees, then he has a potion ready for her to take.  Friar Laurence outlines the plan for Juliet that she should take the potion, which will make her fall into a deep sleep, with her body growing cold, and her breath becoming so shallow as to be unnoticeable. When the Capulets put her in the family burial vault, Friar Laurence will send word to Romeo to come and get her before the potion wears off.  In that way, the two can be together. 

This plan is not merely to help Romeo and Juliet, however.  Friar Laurence could get into a lot of trouble for going behind the backs of the Capulets and the Montagues and marrying the couple without the families' knowledge or permission.  This plot would allow the Friar to escape having to explain his decision, as well as avoiding any possible negative repercussions.  At this point, he is attempting to save himself as well as helping the young couple. 

Unfortunately, "the best laid schemes of mice and men" have a way of not working out.  Lord Capulet moves the wedding from Thursday to Wednesday, shortening the time allowed for Friar Laurence to alert Romeo of the plan, and Friar John, who is supposed to deliver the news to Romeo, gets held up in a quarantine.  Romeo never learns that she has faked her death, and he takes poison next to her "body" in the crypt.  Juliet wakes up to find Romeo dead, and she kills herself with his dagger.