In several episodes of Forever Knight, people refer to "taking the Fifth"; and, in the episode "Spin Doctor", Nick cites it in court in order to avoid testifying at one of the McCarthy hearings in the 1950s. These references are to the Fifth Amendment to the United States Constitution.
The original United States Constitution was adopted on 17 September 1787, by the Constitutional Convention in Philadelphia, Pennsylvania, and later ratified by conventions in each U.S. state in the name of "The People". However, in 1789, a series of articles were introduced by James Madison to the First United States Congress. When they had been ratified by three-quarters of the States, they came into came into effect on 15 December 1791. Collectively, the first ten amendments to the United States Constitution are known in the U.S. as the Bill of Rights.
The text of the Fifth Amendment states:
|“||No person shall be held to answer for a capital, or otherwise infamous crime, unless on presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation.||”|
The portion of this amendment that is relevant to the scenes in Forever Knight is the phrase, "nor shall be compelled in any criminal case to be a witness against himself".
The fifth amendment protects witnesses from being forced to incriminate themselves. To "plead the Fifth" is to refuse to answer a question because the response could provide self-incriminating evidence of an illegal conduct punished by fines, penalties or forfeiture. The fifth amendment protections apply when an individual is compelled to testify at a legal proceeding. They also apply during custodial interrogation, such as being questioned by the police.
It is important to note that the "Fifth Amendment" is part of the American constitution, and hence part of American law. It is not part of the law of any other country. In particular (given that Forever Knight is set in Canada), the Fifth Amendment is not part of Canadian law.