While trying to prove the innocence of mcarthy 's son , sherlock untangles a web of blackmail , gangs and robbery with the " rat " in question being a dying reference to the murderer .

在力证麦克阿瑟儿子清白的过程中，福尔摩斯靠着“rat”这个指向凶手的死亡讯息，破解了一个由敲诈、黑帮和强盗组成的错综复杂大网。