LONDON—In the predawn hours of Christmas morning, a 40-year-old shoe repairman who goes by the name Robbo squeezed his 6-foot-8-inch frame into a wet suit, tossed some spray cans into a plastic bag, and crossed Regent's Canal on a red-and-blue air mattress.

Robbo, one of the lost pioneers of London's 1980s graffiti scene, was emerging from a long retirement. He had a mission: to settle a score with the world-famous street artist Banksy, who, Robbo believes, had attacked his legacy.

The...