Short answers, pulled from the story.
Volume I of Das Kapital was published in September 1867 by Otto Meissner in Hamburg. Karl Marx delivered the German manuscript to Meissner in April 1867. Volumes II and III were published posthumously by Friedrich Engels in 1885 and 1894 respectively.
Das Kapital argues that the motivating force of capitalism is the exploitation of labour. Workers' unpaid labour is the ultimate source of surplus value and profit. The capitalist purchases labour power at its value but extracts from workers more value than they are paid for.
Commodity fetishism, as described in Das Kapital, is the process by which social relations between producers appear as objective relations between things. The products of labour seem to possess intrinsic value, and market exchanges appear to be governed by properties of the commodities themselves rather than by the underlying social relations of production. Marx described this as an objective appearance arising from the structure of commodity production, not merely a subjective illusion.
Das Kapital has three volumes. Volume I focuses on the production process of capital, covering the commodity, surplus value, the working day, and primitive accumulation. Volume II examines the circulation of capital, including the circuits of capital and the reproduction of the total social capital. Volume III addresses the process of capitalist production as a whole, including the rate of profit, interest-bearing capital, ground rent, and the distribution of surplus value.
Marx synthesised three major intellectual traditions in Das Kapital. He critically engaged classical political economy, including thinkers such as Adam Smith and David Ricardo. He drew on German idealist philosophy, particularly the dialectical method of Georg Wilhelm Friedrich Hegel, which he recast in a materialist framework. He also engaged with French utopian socialist thought, including figures such as Charles Fourier and Pierre-Joseph Proudhon, aiming to convert utopianism into what he called scientific socialism.
In Volume III of Das Kapital, Marx argues that the general rate of profit tends to fall in the long run as capitalists introduce labour-saving machinery. This raises the organic composition of capital, meaning more is invested in machinery and raw materials relative to wage labour. Since only living labour produces surplus value, a rising ratio of machinery to labour depresses the rate of profit even as individual productivity increases. Marx identified several counteracting tendencies, including raising the intensity of exploitation and opening foreign trade.