Chasing a vile English plant, Professor Peter Shandy and his friends go on a most peculiar trip.

The giant hogweed, a creeping menace known for crushing the life out of any plant foolish enough to get in its way, has put the hedgerows and pastures of the English countryside in jeopardy. Fishermen find their streams clogged, young lovers are caught with rashes in embarrassing places, and the English nudist colony has been all but exterminated. Only Peter Shandy, the famed horticulturalist responsible for the world's finest rutabaga, can save the day. But when Shandy and his colleagues set out to find hogweed samples, they stumble into an unusually mystical adventure. Quite by accident, Shandy trips through a publican's portal, and finds himself conversing with a giant. Trapped in a land of castles, wizards, and knights, Shandy must use every scrap of his horticultural genius to get back home—lest the hogweed triumph in his absence.