"Portland Street Blues" follows the rise of the legendary triad boss Sister Thirteen (Sandra Ng). We begin in the present day. Thirteen meets with another triad boss Fa-Fit, who wants to take over all operations in Portland Street, the famous Hong Kong Red Light district. Thirteen is threatened from many sides, but still has some support, mainly from the big boss Uncle Pun.
We go back in time to when Thirteen was a teenager. Her father is a avid gambler. He won 13 straight games the night she was born, which is the reason for her name.
Thirteen and her girlfriend - and perhaps lover - Yun (Kristy Yeung) run a little con operation. Yun seduces a man. Thirteen arrives to collect money as her pimp, and while Yun takes the man to a hotel room, Thirteen calls the cops, who busts through the door while Yun sneaks out through the back door (or window).
After they manage to rip off their english teacher this way, they take the winnings and bet them on a boxing match. Thirteen is completely infatuated with one of the boxers Coke (Alex Fong), and she has her picture taken with Coke and Yun. Thirteen keeps the picture close at all times, and she constantly dreams of Coke despite her obvious close relations to Yun
...SPOILER ALERT!!! DON'T READ ANY FURTHER, IF YOU DON'T WISH TO HAVE CRUCIAL PLOT POINTS REVEALED...
One night Thirteen's father wins the lottery, but makes the mistake of telling everybody about it. Shortly after the boss SOB arrives to claim his share (which is 100%!), but the father refuses and everything escalates and he is killed right in front of Thirteen. When she later attempts to get revenge she's knocked down by SOB and his boys, but before they can kill her a girl comes to her rescue. The girl is Kei aka Scarface (Shu Qi), a local junkie. SOB owes her a favour, so he lets Thirteen live, at least for the time being.
The two girls talk and Scarface explains how she fell from grace. She was in love with a cop, and became pregnant. When she approached him with the news, he'd already found a new girl and to make matters worse he kicked her in the stomach, effectively causing an abortion.
Soon after SOB is found killed. He was shot by Coke, who flees to Mainland China. Thirteen races to the harbor to see him, and manages to prevent a last minute assassination attempt, but is hit in the process. Coke escape and takes the wounded Thirteen with him to China. They seek refuge with one of Coke's friends who has a small workshop.
In China Thirteen slowly recovers. Her friendship with Coke develops and she begins to fall in love with him, but before anything can happen, Yun arrives and unintentionally takes Coke's attention, and slowly their secluded paradise disintegrates.
Thirteen returns to Hong Kong and is introduced to the Triad bosses. They have a problem with an undercover cop, who's threatening their operations. The big boss promises that anyone who kills him will get the opportunity to rule Portland Street. When Thirteen learns that the cop is James, Scarface's lost love, she volunteers to handle the problem, and she does so with the help of Scarface.
Thus having come full circle in the flashback, we return to the present, where the conflict between triad bosses is still brewing and where Sisters Thirteen's position in Portland Street is still in danger. But life has made her fearless and resolved. Only betrayal from inside her own ranks can truly threaten her now.