Community ecologists value the phenomenological observation of plant biotic interactions because they provide assumptions to make predictions of other ecosystem features, such as species diversity, community structure, or plant atmospheric carbon uptake. However, a rising number of scientists claim for the need of a mechanistic understanding of plant interactions, due to the limitations that a phenomenological approach raises both in empirical and modeling studies. Scattered studies take a mechanistic approach to plant interactions, but we still lack an integrated theoretical framework to start approaching holistically. In this Review and Synthesis, we present a comprehensive foundation for the study of the mechanisms underpinning the net interaction between two plants. First, we recapitulate the elementary units of plant interactions, i.e. all the known biophysical processes affected by the presence of an influencing plant and the possible phenotypic responses of influenced plants to these processes. Following, we discuss how a net interaction between two plants may emerge from the simultaneous effect of these elementary units. We then touch upon the spatial and temporal variability of this net interaction, and scrutinize how that variability may be linked to the underlying biophysical processes. We conclude by arguing how these processes can be integrated in a mechanistic framework for plant interactions, and why it must necessarily focus on the individual scale, incorporate the spatial structure of the community, and explicitly account for environmental factors.