Johan Padan and the Discovery of America, Riverside Studios, London

It's not easy to be outspokenly left-wing in Italy, where fondness for fascism is more than nostalgic. Conversely, though, it's easy for a dissenter to win a reputation. Since others have won the Nobel prize for virtue or suffering rather than literary talent, one can hardly begrudge Dario Fo his award, especially since he gave the half-million pounds away. Considering Johan Padan and the Discovery of America by English rather than Italian or Swedish standards, however, one is impressed neither by its bravery nor its beauty but, like the actor who delivers it, its fatuous, bullying geniality.

Like those men in a bar who just will not go away, Mario Pirovano frequently nods and takes one step back, but his teeth take one step forward. Grinning at the mild applause ("Is lovely"), and asking our indulgence, he says, "I'm not very good at English". Actually, Pirovano, who translated the play, is more fluent in our language than many native speakers. In his chats before and after the play, though, his English falters when he says that he is aware of our reactions ("I can feel some of you sometime") or that he is returning theatre to its ancient ways of "people alone on a stage with nothing on". These clearly intentional funny-foreigner mistakes reinforce the dishonest, laugh-cadging nature of the script.

Johan Padan (always referred to, in traditional cod-mythic fashion, by his full name) is an Italian who, fleeing persecution at home, then in Seville during the Inquisition, stows away with Columbus. Put to work with the animals in the hold, his voyage is stormy and unpleasant ("The wave goes plaff! The horse goes plof!"), but the New World is ample recompense, with friendly naked natives who invite him to join them on their beds of leaves ("single, small double, king size").

The snakes in this lovely garden are the Spaniards, who steal from, rape, and enslave the Indians, a gentle, free-loving lot who enjoy the occasional "joint" and whose only vice, a bit of cannibalism, has the sanction of natural economy – when the natives hear about the Inquisition, they decry the terrible waste of roasting people and not eating them. Johan Padan becomes a hero to the natives when, with a needle he has somehow kept after losing all his possessions, he sews up several wounded villagers, including their shaman, who has been cleft stem to stern. They also seem to enjoy, as Italian audiences clearly do, his incessant ball and bum jokes. Pirovano's expressions and his movements – crude mimes of swimming, dancing, stitching – merely illustrate rather than colour the story.

One can at least enjoy the lovely backcloths painted by Fo, one of Renaissance-style demons torturing naked sinners, the other of a jungle paradise. But his play is only a long and self-regarding restatement of this simple point of view.

To 8 June (020-8237 1111)