Roberta's Blood Trail is a 5 episode OVA spin-off of Black Lagoon, featuring all the the main characters, as well as the psycho ex-FARC maid Roberta. Roberta is on the warpath and comes to the fictional Thai city of Roanapur (hive of scum and villainy) seeking revenge. After wiping the floor with scores of criminals, the local crime lords and FARC move against her, while Garcia tries to persuade the Black Lagoon company to help him get her back.
The storyline was more confusing than it deserved to be, and Rock goes out-of-character crazy for part of it. I know that Black Lagoon is all about laughing insanely but here it was a bit overblown, as Rock's plan isn't all that fancy anyway. Roberta is an ambivalent figure throughout, as she has essentially gone berserk; I found Fabiola (played by Yukino Satsuki) irritating at first, but she grew on me.
Roberta's Blood Trail is full of action and gunfights as series fans would expect, though Revy (and most of the Black Lagoon crew) plays a surprisingly small part in that.