Blood Tide is the first episode of Season 4.
In 1202, an army led by the three vampire elders (Markus, Viktor, and Amelia) arrives at a village that was attacked by pureblood werewolves. Viktor and Amelia capture their target, Markus's twin brother, William Corvinus, the first and most powerful werewolf. Despite Markus's defiance, Viktor orders that William be imprisoned in a secret location forever.
In the present day, vampire Selene takes Michael to a safe house so that she can confront the vampire regent Kraven; she knows that Kraven intends to kill Markus and plans to stop him. However, Markus has awakened before Kraven arrives. Markus kills Kraven and his men. Lorenz Macaro, an elderly man, sends in a team of "cleaners" to investigate the aftermath from the battle in the Lycans' lair. When Macaro examines Viktor's corpse he finds a metal disc inside which is the match to a pendant originally worn by Sonja. The other half of the pendant is now possessed by Michael after the death of Lucian.
Markus tracks Selene and Michael down and attacks them, but they evade him and hide in a warehouse. There, Selene seduces Michael and they share their feelings for one another by stripping the other of their clothes and have passionate sex at a warehouse inside a cargo container.
Now knowing that the pendant is important to Markus, Michael and Selene set out to discover why Markus wants it. Selene recalls that she saw it as a child, but does not know its significance. They travel to the hideout of the exiled vampire historian Andreas Tanis.
Tanis reveals that Markus was the first vampire, one of the three sons of Alexander Corvinus, the first immortal. Markus was bitten by a bat and became a vampire, while William was bitten by a wolf and became a werewolf. The third son remained human and gave rise to a line of mortal descendants including Michael, who became the first Lycan–Vampire hybrid. The first werewolves created by William were entirely animal and unable to return to human form. Due to William's destructiveness, Markus approached Viktor, then a dying mortal warlord, and offered to turn him and his army into immortal vampires in exchange for tracking down and stopping William, and in destroying those he had infected. Viktor did not kill the brothers because he was led by Markus to believe that doing so would result in the immediate extinction of all other vampires and his Lycan slaves. Tanis further reveals that Selene's father was the architect who built William's prison and that the pendant is the key. Viktor killed Selene's family because they knew the prison's location, but turned Selene into a vampire with the location of the prison encoded in her blood. Tanis then refers Selene and Michael to Macaro for help. After Selene and Michael leave, Markus arrives and drinks Tanis' blood to learn Selene and Michael's location, killing Tanis.
Visiting Macaro, Selene and Michael discover he actually is Alexander Corvinus. Alexander reveals that he has devoted his entire immortal life to keeping the Vampire-Lycan war a secret. However, he refuses to assist Selene in killing his sons. Then Markus arrives, impales Michael and learns the location of William's prison by drinking Selene's blood. He mortally wounds his father and obtains the other half of the pendant. He mocks his father's refusal to help William, revealing that he intends, with William, to rule the world as god-like master of a race of vampire-Lycan hybrids. On Alexander's bidding, Selene drinks his blood, enhancing her physical strength and healing abilities to a level equivalent to that of a hybrid. Afterwards, Alexander blows up his ship, killing himself.
Selene, aboard Alexander's helicopter, leads his cleaners to the prison to confront and destroy Markus, but he has already freed William. In the ensuing battle, William bites the soldiers, who then turn into werewolves and proceed to physically overpower Selene. Michael, presumed dead and carried aboard the helicopter, awakens and joins the fight in his hybrid form, killing William by ripping his head off. Selene engages Markus in hand-to-hand combat, killing him by kicking him into the rotor blades of the crashed helicopter.
After the battle, Selene realizes that Alexander's blood has made her have human abilities including being immune to the effects of sunlight, which is lethal to vampires.