A global array of proxy records suggests that the 4.2 ka event marks an ~ 300-year period of major climate anomaly. However, the climatic manifestation of this event, including its onset, duration, and termination, remain less clear in the Indian summer monsoon (ISM) domain. Here, we present a new speleothem oxygen isotope (δ 18 O) record from Mawmluh Cave, Northeast India, which provides an annually-resolved record of changes in ISM strength between ~ 4.440 and 3.780 ka BP. Our δ 18 O record is constrained by 18 230 Th dates with an average age uncertainty of ±13 years and a dating resolution of ~ 40 years, which allow us to characterize the timing and structure of the 4.2 ka event with an unprecedented detail. The overall expression of the 4.2 ka event in our record shares broad similarities with a previous δ 18 O record from the Mawmluh Cave as well as with other previous lower-resolution proxy reconstructions of the ISM. However, unlike some previous ISM records, where the 4.2 ka event has been described as a singular multi-centennial period of anomalously weak ISM, our data suggest a more variable response of ISM during this period. The 4.2 ka event in our record exhibits a three-stage structure, characterized by highly variable ISM between ~ 4.255 and 4.070 ka BP and a distinct pluvial phase from ~ 4.070–4.010 ka BP. The latter abruptly (< 10 years) culminated into a relatively weaker phase of ISM, which was punctuated by a number of multidecadal periods of anomalously drier conditions. While, our record shows evidence of a discernible beginning for the 4.2 ka event, there is no clear evidence of its end in thus, suggesting that the ISM experienced a major regime-shift or transition at ~ 4.0 ka BP.