Peregrine is a young man who reads too much. His head is full of blue elephants, woolly chickens and dog-headed men, put there by Sir John Mandeville, the bogus knight whose medieval travelogue made such a mark on the English imagination. Peregrine is so desperate to follow in his hero's footsteps that he won't even consummate his marriage. After three years, his wife is ready to climb the walls.

Peregrine's father, Joyless, calls in the 17th-century equivalent of a psychiatrist, who resolves to cure the madness by faking a trip to the other side of the world with the help of a band of actors. And so begins a long play-within-a-play that immerses Peregrine in an "anti-London" where cats must be protected from mice, prostitutes are pillars of society, and sick men give healthcare tips to doctors. Meanwhile, Doctor Hughball attempts another cure: that of Joyless, who is terrified his own young wife will cuckold him.

The Antipodes, or The World Upside Down, first saw the light in 1638, but has not been staged professionally for 350 years. Its author was Richard Brome, Ben Jonson's former servant, who produced a string of comedies such as A Joviall Crew and The Sparagus Garden. But while Shakespeare's Globe describes this play as a "neglected masterpiece", that's only half true. The Antipodes is an enjoyable mingling of broad humour and social critique, but it milks its few ideas for all they're worth, and demands faultless acting and direction if it is to rise above the mass.

Instead, what we get is a partial success. The play-within-a-play is hilarious, thanks to the actors-playing-actors, who seem to be having the time of their lives. Geoffrey Beevers as the neat, self-satisfied Hughball, Penny Layden as Joyless's wife, Diana, and Mark Lockyer as Byplay, the ever-resourceful chief actor, all give first-rate performances. But director Gerald Freedman appears to have forgotten that, in a comedy, there's no point reconciling estranged couples unless the audience want to see them together.

There's little he could do with the pairing of fresh and devoted Diana and dry old Joyless (blame Brome's text); but Freedman and his cast badly misjudge the younger couple. Peregrine (Harry Gostelow) and his wife, Martha (Karen Tomlin), should be affecting as well as ridiculous; instead, he is so insipid and she so brainless that you couldn't give a damn whether they ever get it on.

Until September 22. Box office: 020-7401 9919.