Decidedly minor stuff for such a major filmmaker, but there's something remarkable about watching a master like Johnnie To reverse-engineer an entire thriller from one idea he just had to try - in this case, a shootout unlike any you've seen before.

Director To fills in the messy back story neatly as he introduces the supporting players, who provide comedy or menace (and sometimes both).

To trusts his actors, and uses them so well that you often forget that we don't know much about them beyond the worried look in their eyes, the smirk on their lips and the arch of their brows.

The director keeps a tight rein on proceedings, with Zhao making the biggest impression as a dedicated but believably flawed medic.