This paper reveals the theoretical framework and practical experience of Ernesto Guevara, investigating how he got involved in the specific task of building socialism in Cuba. Guevara studied Marxist literature extensively, and the changes experienced by socialist bloc countries in the 1960s. He elaborated the Budgetary Finance System which was opposed to the auto-financing defended by Carlos Rafael Rodrígues. This opposition was embodied in the Great Economic Debate. A crucial point of the discussion was the view on planning and the law of value. For Guevara, the latter could not be used to build socialism, whilst centralised planning was the ‘defining category’ of socialism. Explaining the context and the main measures of the New Economic Policy (NEP) applied in Soviet Russia in 1920s, this paper investigates Guevara’s critical view on it, based on his methodological approach to Das Kapital. Guevara revealed a link between the NEP and market experiments of the 1960s, forecasting a return to capitalism in the USSR. Guevara’s radical political economy is crucial for understanding his legacy holistically and may be considered a message to the countries and social movements of the Global South in their struggle for socialism, warning them away from distortion of Marxism.