For some reason, he is in the Soho district in New York. It turns out that Marx was allowed to leave heaven. Yes, in this play Marx ends up in heaven rather than the hotter spot. Heaven is run by liberals who don't care if he yaks on, as long as Marx doesn't sound too radical. Marx is determined to explain why the gulags in the former Soviet Union weren't his fault, eager to condemn the inequities of capitalism.
Dreamland is more like it.
But Zinn, in an effort to make Marx sympathetic, airbrushes his thorny personality. Zinn misses the man's megalomania, which was mitigated by a savage self-mockery. As for Marx's politics, Zinn's message is that Marx was more of a humanitarian than people give him credit for. At one point, Marx remembers a hanging he witnessed in 19th century Soho, in London. But that doesn't free Marx of complicity with the crimes of the gulags.