We characterize inﬂation dynamics during the Great Lockdown using scanner data covering millions of transactions for fast-moving consumer goods in the United Kingdom. We show that there was a signiﬁcant and widespread spike in inﬂation. First, aggregate month-to-month inﬂation was 2.4% in the ﬁrst month of lockdown, a rate over 10 times higher than in preceding months. Over half of this increase stems from reduced frequency of promotions. Consumers’ purchasing power was further eroded by a reduction in product variety, leading to a further 85 basis points increase in the eﬀective cost of living. Second, 96% of households have experienced inﬂation in 2020, while in prior years around half of households experienced deﬂation. Third, there was inﬂation in most product categories, including those that expe-rienced output falls. Only 13% of product categories experienced deﬂation, compared with over half in previous years. While market-based measures of inﬂation expectations point to disinﬂation or deﬂation, these ﬁndings indicate a risk of stagﬂation should not be ruled out. We hope our approach can serve as a template to facilitate rapid diagnosis of inﬂation risks during economic crises, leveraging scanner data and appropriate price indices in real-time.