user: Question: When 00:21 is the total footage (mm:ss) what is the serial?
Columns: ['Doctor', 'Season', 'Story no.', 'Serial', 'Number of episodes', 'Total footage remaining from missing episodes (mm:ss)', 'Missing episodes with recovered footage', 'Country/Territory', 'Source', 'Format', 'Total footage (mm:ss)']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['First', '1', '008', 'The Reign of Terror', '6', '00:21', 'Episode 4', 'Australia', 'Private individual', '8mm cine', '00:10']
SQL:

assistant: SELECT "Serial" FROM "Table" WHERE "Total footage (mm:ss)" = '00:21';