The untwisting of the helical structure of a chiral liquid crystal (CLC) in a thin plane layer exposed to an external action (temperature or field) and its dependence on the molecular adhesive forces at the layer boundaries are studied theoretically. It is shown that the critical electric (magnetic) field for complete untwisting in a thin layer may be appreciably lower than in the corresponding bulk CLC sample, and, contrary to the latter, the untwisting proceeds jumpwise. The expressions relating the jump temperature (field), i.e., the magnitude of untwisting action, to the CLC material parameters, layer thickness, and surface adhesive potential are given. The jump temperature (field) hysteresis is studied. In particular, it is shown that, for certain parameters, the untwisted helix remains untwisted after the removal of external action. The revealed qualitative regularities of untwisting are illustrated by numerical computations with the use of particular parameters of a CLC layer.