The Brexit saga has reached a watershed moment. The United Kingdom withdrew from the European Union on 31 January 2020 and, following the expiry of the transitional period laid down in the EU-UK Withdrawal Agreement, it ceased to be bound by EU law. By the same token, it entered unchartered waters as a former EU Member State trying to find its place in an economically integrated world. This article takes stock of the legal affairs as they stood on 1 January 2021. Yet, at the same time, it puts the new EU-UK legal framework in a broader perspective. For this purpose, it treats as a point of reference the Political Declaration, which was signed alongside the EU-UK Withdrawal Agreement. A good chunk of its potential has materialised in the EU-UK Trade and Cooperation Agreement, although in some respects the proposals laid down in the Political Declaration are yet to turn into reality. Thus, to confine it to history books would be rather premature. While it is impossible to predict the future, the time is right to put the EU-UK legal framework under the microscope and to analyse its main legal parameters. The present article offers such an insight. In part I, the centre of gravity is on institutional matters.