The young, bright and decent London tramp Alan Terry accidentally witnesses a mob execution. He gets away, but leaves enough evidence for them to come looking for him among the homeless, and most violently: his older best friend is killed by arson. It turns out that Matheson, the police Detective Inspector in charge of the murder investigation, is in cahoot with the mob boss Adolfo Cavani, and actually on Alan's trace, with the firm intent to eliminate the dangerous witness. The only one who may actually help Alan is a female reporter...