From the later career of the Baron, he's now an established art dealer occasionally using his skills to help other people solve mysteries that are somewhat outside the skills of the police.
He's summoned by an older woman to look at some paintings in her attic, when he gets there she's been attacked and her house is on fire, he sends the paintings to the police and proceeds to investigate the situation, his interest piqued. It's complicated and messy and Mannering is apparently having a ball of a time.
It's not a scenario that would work today but it's an entertaining snapshot of the time and a messy mystery that is quite a lot of fun. ( )