The article provides a rational reconstruction of the unbeatable fear excuse under the Chilean criminal law system. This objective is developed based on the verification of a special treatment that Chilean jurisprudence gives to this criminal excuse, according to two special normative elements: irresistible impulses and mental disorders. This rational reconstruction only allows real evils in the unbeatable fear excuse. For this reason, the central objective is to demonstrate how the unbeatable fear excuse only recognizes mental disorder that does not affect the agent reality judgment. Also, this article analyzes the effects of this doctrinal interpretation. Thus, according to the necessity defense and the recognition of a certain type of mental disorders under the unbeatable fear excuse. Finally, this article tries to make a dogmatic differentiation between perceptive and imaginary acts under the unbeatable fear cases. © 2019 Centro Estudios Derecho Penal. All rights reserved.