His plan is to hide in the tomb with Juliet. That should have been his plan all along, and he should have got on it sooner.
The Friar's plan is to hide Juliet in the Capulet family tomb, where Romeo will later come to retrieve her once she awakens from the potion-induced sleep.
At Friar Lawrence's
wait
The Friar's plan in Act III Scene 2 is not very detailed but does calm Romeo down a bit. The Friar says, first, consummate the marriage. Romeo can do that because his banishment does not start until the morning of the next day. After spending the night with Juliet, Romeo is to go bright and early to Mantua and . . . well, the friar is not very clear here about what happens next. Basically, Romeo is to hang around in Mantua until the friar figures out some way to get him together with Juliet. The Friar does not actually have a plan, but he is buying time.
In the end of the scene, Friar Lawrence gives Romeo a plan to reunite with Juliet by faking his own death and then running away together once he is safely in Verona. This plan involves taking a potion that will make him appear dead for 42 hours and then sending a letter to inform Juliet of the situation.
The Friar goes to Juliet's tomb to retrieve her, after being informed that Romeo never got the letter telling him to fetch Juliet. He planned on getting her out, and letting her stay with him until he could write another letter to Romeo telling him to come get her. As you probably found out, that didn't happen as planned.
Friar Laurence hopes that if he marries Romeo and Juliet that the feud with their families with end. Little did he know that will not happen until they are no longer living.
Friar Lawrence went to the tomb to get Juliet out of there because she was about to wake up and he did not want to leave her awake, surrounded by corpses. He was going to bring her to his cell until Romeo can come and get her.
Friar Laurence gives Juliet a potion at the opening of Act 4. It is the only way she can avoid being bigamously married to Paris. It is not a poison, however, just a Mickey Finn designed to knock her out for a couple of days.
Romeo's part in the Friar's plan is to fake his own death by drinking a potion that will make him appear lifeless. The Friar's plan is for Romeo to be placed in the Capulet tomb until Juliet can join him there after faking her own death.
Friar Lawrence tells everyone the truth including the Prince, but unfortunately doesn't do it until it is too late.
Friar Lawrence advises Romeo to go to Mantua until things have settled down in Verona. He believes that staying away for a while will help defuse the tension and allow for a better outcome for Romeo and Juliet.
In "Romeo and Juliet," Romeo and Juliet secretly make plans to marry with the help of Friar Laurence. Their intentions are not publicly announced to their families until after they have already been married in secret.