This article tells the story of customary international law in the 98 years between the introduction of the draft rules to the new Permanent Court of International Justice and the adoption of the Conclusions on the identification of customary international law by the International Law Commission in 2018. This story is articulated around four moments of rupture and metamorphoses: 1920, 1927, 1986, and 2018. Each of these four metamorphoses originates in powerful interventions by actors resulting in a redefinition of how arguments about the customary status of a rule ought to be made. It is argued that the doctrine of customary international law, by undergoing these four metamorphoses, has gone through 4 different stages: the age of innocence (1920–1927), the age of dualism (1927–1986), the age of turmoil (1986–2018), and the return to innocence (2018-present). The story offered in this article is a story about the four lives of customary international law.