The Prisoner

The "truth serum" being given to Rabb is likely sodium thiopental or amobarbital, which are barbiturate-class sedatives that were once used for general anesthesia and are sometimes still used to treat seizures and used for lethal injections and euthanasia. When used while interrogating a person the drug does not literally force a person to tell the truth, it just makes it difficult for them to lie. A few of the effects of barbiturates are causing a relaxed state of euphoria, making a person more compliant, and suppression of higher cortical brain function; lying requires complex thought processes, with the drug suppressing higher cortical functions it makes it difficult for a person to concentrate on making up a lie, whereas telling the truth simply requires stating information already known and doesn't require higher cortical function. A lot of countries no longer rely on truth serums as most psychiatrists agree that the information extracted from a person in the highly suggestible state these drugs put them in is unreliable at best; in fact most countries justice systems, including the U.S., have deemed testimony based on drug induced interrogations to be coerced and not admissible in a court of law.

When Meg and Alison find Harm's billfold and look at his ID card, that card shows his rank as lieutenant (rather than lieutenant commander), and it shows a social-security number different from the one which he recites to his Chinese captors.