You Don't Know What The Truth Is

You don't know what the truth, the whole truth and nothing but the truth is.

plural truths

1 a archaic : fidelity, constancy b : sincerity in action, character, and utterance2 a (1) : the state of being the case : fact 

(2) : the body of real things, events, and facts : actuality (3) often capitalized : a transcendent fundamental or spiritual reality b : a judgment, proposition, or idea that is true or accepted as true <truths of thermodynamics> c : the body of true statements and propositions

1. Do you swear to tell the whole truth and nothing but the truth?
2. Her story contains a grain of truth but also lots of exaggeration
   

the truth - what the witness experienced
the whole truth - not leaving any material out
nothing but the truth - definitely no lies

The dictionary definition of “fact” is, “Knowledge or information based on real occurrences,” and the source I consulted went on to delineate the term as used in Law, namely, “The aspect of a case at law comprising events determined by evidence: The jury made a finding of fact.” This last leads us to a dictionary definition of truth, which is, “the property (as of a statement) of being in accord with fact or reality,” which explains why all who testify in legal proceedings are required to take an oath swearing “to tell the truth, the whole truth, and nothing but the truth.”
A partial truth is just as much a lie as something fabricated from whole cloth.