TODAY'S HEROES are sometimes tomorrow's villains, and unfortunately, to a limited extent, that is the unhappy lot of Antonio Buero Vallejo, whose play Historia de una escalera (Story of a Staircase) in 1949 injected new life into the stagnant Spanish theater and inspired many young dramatists to write serious plays, directly or indirectly criticizing the political, social, and economic policies of Spain. A prisoner for six and a half years, under sentence of death for eight months, Buero was released in 1946 and soon after won the Lope de Vega prize for Historia de una escalera, which became an instant hit. Within several years, he and another outstanding young dramatist, Alfonso Sastre, became symbols of liberal opposition to the Franco regime. liberal opposition to the Franco regime.