Yes, you must appear before the grand jury. You can be arrested if you fail to appear.
You will not be able to escape the grand jury subpoena by simply "Pleading the 5th". In order to plead the 5th, you must actually have a valid 5th amendment privilege.
So, what is a 5th amendment privilege? A 5th amendment privilege protects a person from saying something that could incriminate him or her. In other words, prosecutors (or police, judges, other government agents) may not force a person to testify if there is a possibility that the person's testimony will get him/her in trouble.
For example, the prosecutors believe that you have information about a homicide. They summons you to appear before a grand jury. Begrudgingly, you come to court. You have zero interest in testifying; you are fearful of retaliation, worried of looking like a snitch, and you just don't want to get involved. So, you tell the prosecutor - "I don't have to testify, I have a 5th." At this point the prosecutor (after rolling her/his eyes) will report to the Superior Court judge that a witness has a potential 5th and that she/he needs an attorney. Hint: consult with an attorney before you show up to court.
You meet with an attorney; the attorney will explain to you the law and the process of "asserting the 5th." Next, you will need to tell your story to the attorney; where you were, what you were doing, and whom you were with. Worry not, the content of this conversation is protected by attorney/client confidentiality. Telling the attorney a full and truthful story is crucial as it allows the attorney to determine if you have any exposure to a criminal charge. If the attorney determines that there is a "real risk that [if you were to testify] your answers would tend to indicate your involvement in illegal activity[,]" Commonwealth v. Martin, 423 Mass. 496 (1996), then your attorney will be able to make the argument to the judge that you "have a 5th." The prosecutor is not present when your attorney explains your 5th amendment privilege to the judge.
If the judge agrees that you have a valid 5th, then the prosecutor may not force you to testify...yet.
Wait, what?! I thought I was done with this; now you're telling me they can still make me testify?
Yes, prosecutors possess one more weapon - Immunity.
What's "immunity?" Immunity is, figuratively speaking, a protective blanket given to people that insulates them from prosecution. A prosecutor may request immunity. The procedure is tedious and too long to explain here; just assume that they will be able to give you immunity. Once you are given immunity you may not be charged for the crime that formed the basis of your 5th. With immunity the threat of self-incrimination no longer exists; therefore, the prosecutor can now compel you to testify. Note: prosecutors do not always pursue immunity; the procedure is burdensome and time-consuming. If your testimony is not particularly important, they may simply drop the issue. However, they are likely to follow through with an immunity request if your testimony is crucial.
So, we went through all of this trouble only to have the government force me to testify? This frustrates me - I am no better off then I was at the beginning. What if I refuse to testify? The answer: you can (and probably will) be found in "contempt." G.L. c. 233, s. 20H. If found in contempt you can be held in jail for up to one year, or until you comply with the order to testify.
In my practice I have found that individuals whom are compelled to testify are just as anxious and worried as those charged with a crime. It is very helpful to have an attorney shepherd you through the process. In addition to fighting to keep you off the stand, an attorney communicates with the prosecutor, helps schedule dates for your testimony and accompanies you to the grand jury room. Note: In Massachusetts state courts your attorney may appear with you in the grand jury room. In Federal court, your attorney may not appear with you in the grand jury room.
Attorney Henry Fasoldt frequently represents people whom are subpoena'd to testify before Grand Juries. You should discuss your situation with a lawyer before responding to a subpoena. Call Attorney Fasoldt if you have received a grand jury subpoena - (617) 338-0009.