This film takes place a few months after Justice League (2017). Crime has risen in Gotham during Batman's absence who was dealing with Steppenwolf and Parademons. While Bruce was away a new shadowy criminal called The Red Hood has violently made his debut by attacking a mob meeting and demanding their loyalty to him. Red Hood shows himself to be an expertly trained marksman and hand-to-hand combatant. However Red Hood boasts hubris which could be his downfall in the future.
Bruce who was made aware of the Red Hood's presense decides to hunt him down and bring him to justice. However other crime lords like Black Mask and The Joker are in the middle of a gang war for majority control of the Gotham underworld. This is the perfect opportunity for The Hood to take out both crime figures so that he can have dominant control over all the smaller gangs in Gotham.
Dick Grayson comes to Gotham to help Bruce with the rising threat in Gotham despite Bruce's rejections. Barbara Gordon who is now Oracle is helping both of the heroes via communications to help take down Red Hood, Joker, and Black Mask combined. But of course this was no easy task. After Black Mask knows that he's being gunned for he hires the most deadly assassin in the world for his protection who's named Deathstroke.
Slade Wilson aka Deathsroke is hired by Black Mask to take out the pursuing threat of Red Hood, Batman, Nightwing. In the DCEU Slade is in charge of a private military company known as Ravager Inc. This company is the most hired pmc in the world and operates internationally. Slade is a resilient supersoldier who's a force to be reckoned with in the battlefield.
Red Hood is armed with a massive armory of firearms, explosives, and gadgetry to go up against all his foes in Gotham. Red Hood wishes not to harm Batman and Nightwing but will incapacitate them if they get in his way of his bloody justice. Red Hood is later revealed to be Jason Todd after Bruce deduced his identity. it's a mystery to the Bat Family on how Jason is alive (Lazarus Pit) but will investigate that later.
Harley Quinn is present in the film to help out Joker in his survival of the gang war. She's unpredictable and is always loyal to Mr. J.
In this film Joker is more sadistically violent than he was in Suicide Squad and will stop at nothing to be king of Gotham.
In the heart of the film this is a Father-Son story mainly between Bruce Wayne and Jason Todd. Bruce wishes to save Jason a second time from being in complete darkness. But Jason doesn't was Bruce saving him he wants Bruce to rectify his mistake and be a killer again once more to take out the joker. But will Bruce regress to his old ways or will he pioneer forward to value all human life.