Based on the romance manga "Koiko no Mainichi " by George Akiyama. 
Shinjuku gangster Sabu and his young wife Koiko try to live a normal life, despite the interferences of gang politics and criminal deals. After she saves his life, Sabu's gang-boss, Tominaga, falls in love with Koiko, and sends Sabu into increasing dangerous situations, hoping to cause his arrest, and thus obligate himself to "take care" of Koiko while Sabu is in prison. 
(Source: AniDB)