With World War II raging across Europe, death and chaos hung in the air. It was a perfect storm of conflict and blood to bring about the Dark Lord Dracula... almost as if it were planned. And it was, by the evil Countess Elizabeth Bartley, niece of Dracula and his willing lieutenant in the eternal battle against humanity. She worked, behind the geo-political scenes, to foster anger and distrust, creating the right elements that could cause a literal World War, and with the violence and death this caused, used this to raise the blood magic needed to bring about Dracula's resurrection.
Thankfully, standing in her way, were two heroes: John Morris, wielder of the Vampire Killer, and Eric Lecarde, who possessed the magical Alucard Spear. Each of these heroes had their own motivation to rid the world of the darkness cast across it -- John wanted a piece of Dracula after the vampire killed his father, while Eric was looking for vengeance against Countess Elizabeth Bartley, the woman that killed Eric's lady-love, Gwendolyn. The two teamed up to work together and fight side by side through Dracula's domain.
At Dracula's castle, though, they found no evidence of the Dark Lord. Instead, the castle was but the first leg of a journey across the continent, as the Countess Bartley fled to other safe havens to sow discontent and plot world domination. This took the heroes through many famous locales, while the scenes of battle and war raged in the distance. In each country the heroes found a new threat left for them, a dangerous set of creatures and traps designed to slow, and hopefully kill, the heroes so they would finish their quest. And yet, at each turn, the heroes did survive, proving more capable than the dangers in front of them.
It was at Castle Prosperina, the last leg of their quest, that the heroes came face to face with the Countess. This castle was a edifice of evil, full of it's own traps as well as several nasty encounters. The heroes were forced to take on Death, who made them also fight each of the previous foes they'd already bested on their quest, all before finally taking on the Grim Reaper himself. Then they had to battle Bartley who had her own set of surprises for the heroes, making for a grueling gauntlet of dangers that never seemed to let up. But even with Bartley's death, their quest wasn't truly at an end.
The resurrection of Dracula had proven successful as the Dark Lord stood at the top of this castle, ready to spread his evil across the whole of the world. The two heroes, Morris and Lecarde, had to face off against the Prince of Darkness for the fate of the world (and, maybe, to ease the strife that lead to the Great War). It was good versus evil for the fate of the world, and thankfully good won out as the heroes were able to defeat Dracula and send the undead creature back to his grave once more. The world had suffered enough but, thanks to the work of these two men, the suffered ended... at least for a time.
Castlevania Bloodlines isn't just a sequel to the Castlevania series, but also a sequel to Bram Stoker's original novel, Dracula. John Morris, hero of the game, is the son of Quincey Morris, one of the side characters in the novel (who is there at the big fight against Dracula at the end of the book).
Meanwhile, much of Eric Lecarde's back-story is covered in Castlevania Judgment. In his youth he was given the Alucard Spear by (of all people) Alucard. Dracula's son forged the spear to aid the Vampire Killer; the two weapons could be used in a battle together to defeat Dracula, which more or less happens (in continuity, anyway -- in the game you play as either hero, not both at once, so in-game it's more of an "instead-of" proposition instead of an "along with" deal). After Eric's era, though, the spear is never really heard from again (aside from some non-continuity appearances).