Brexit has brought tensions in European and (especially) British politics. This article illustrates the rationale, scope and research questions of the special issue, which investigates the first Brexit effects in the five years following the 2016 referendum. Taking the distribution of political power as our primary focus and analysing mainly – though not exclusively – British politics, we trace the first developments in the three domains of politics, polity and policy since the UK’s decision to leave the EU. In the politics domain, after the political uncertainties surrounding the referendum period, we detect a return to the power-hoarding dynamics typical of the Westminster model. However, the territorial and constitutional architectures of the British polity are under considerable strain, with Brexit strengthening the nationalistic movements in Scotland and Northern Ireland. In the policy domain, despite strong common interests, Brexit has failed to produce cooperative EU–UK arrangements in finance and foreign policy.