Although inter-organizational information systems (IOIS) implementation has been widely studied, mainstream literature has not focused on understanding how implementation unfolds and how the existing components of the installed base shape the process. This paper addresses this gap by conducting a socio-technical, process-oriented, and multi-level study. Based on a longitudinal in-depth case study of the implementation of an industry IOIS, we develop an explication of IOIS implementation that considers the role of the installed base. Using the lens of actor-network theory (ANT), we counter the mainstream IOIS literature by showing that IOIS implementation cannot only be explained by a fixed set of independent factors; instead, the dynamic mutual shaping of socio-technical actors throughout implementation complements existing factor-based models in explaining the evolution and the outcome (success or failure). The study also shows the importance of complying with the technical and non-technical components of the installed base for an IOIS to be successfully initiated.