Spike was once a mere poet from London named William, he was called "William the Bloody" because he "wrote such bloody awful poetry." He fell in love with a wealthy woman named Cecily, who rejected him since she felt that he was beneath her. On his way home, he met Drusilla, who transformed him into a vampire. He joined up with her, Angelus, and Darla. William the Bloody soon earned the name Spike, for torturing his victims with railroad spikes.
Spike soon grew obsessed with the Slayer and was determined to kill one. He met his first Slayer during the Boxer Rebellion and killed her (earning a scar over his eyebrow in the process). In the 1970s, he met another Slayer in New York City and killed her on a subway train, taking her leather duster as a souvenir. In 1998, he and Drusilla fled from Prague after she was nearly killed by a mob and came to Sunnydale to heal her. He killed the Anointed One and took over as the new leader of the Master's legions. He became obsessed with killing the current Slayer, Buffy Summers, but repeatedly failed to do so. He later captured Angel and proceeded to conduct a ritual to heal Drusilla. In a battle with Buffy, he was left crippled and confined to a wheelchair. He and Drusilla arranged to reassemble the Judge and destroy the world on Drusilla's birthday, and were later rejoined by Angelus.
Spike grew more and more angry with Angelus, for he felt he was leading them to their doom by tormenting Buffy rather than killing her. He also began to believe that Angelus and Drusilla were having an affair. Spike soon regained his ability to walk, but hid it as he planned to eventually kill Angelus. Some time later, the statue of Acathla was discovered, and stolen by Angelus and Drusilla. Spike knew he had to act before Acathla was awakened, and sucked the world into Hell. Spike liked the world- in his own words: "You've got dog racing, Manchester United, and you got people. Billions of people walking around like happy meals with legs." He convinced Buffy to help him in exchange for helping her rescue Rupert Giles while he and Dru skipped town. Spike incapacitated Dru and took her away, leaving Buffy to die at Angelus's hands. They arrived in Brazil, where Dru began to sense his growing infatuation with Buffy, which he did not acknowledge having. They broke up when he caught Dru making out with a Chaos Demon on a park bench.
Spike returned to Sunnydale to take revenge on Angel, but decided to force Willow Rosenberg to concoct a love spell he could use on Dru instead. He kidnapped her and Xander Harris to ensure she'd do it. After beating up a group of vampires sent after him by Mayor Wilkins, and watching Buffy and Angel, he decided to just find Dru, tie her up and torture her until she liked him again. This turned out to be unsuccessful, and he returned to Sunnydale a year later.
Now, he was seeking the Gem of Amarra, which would make vampires invincible. Spike hooked up with the newly vampirized Harmony in the process. After finding the Gem, he ditched Harmony and fought Buffy, but lost the Gem in battle. He pursued the Gem to Los Angeles. Spike captured Angel and had him tortured to find it, but it was instead taken by the torturer he hired. He returned to Sunnydale to kill Buffy but was captured by the Initiative. They implanted a chip in his brain, preventing him from harming humans. He was also designated as Hostile 17.
Spike escaped and later ran to Buffy for help, where they kept him tied up at Giles' house. He was later forced to move into Xander's house, and then moved into a crypt (stealing some of Xander's possessions when he did so). When he discovered he could still kill demons and vampires, he took great pleasure in it and began to kill them as he encountered them. He continued to sell information to Buffy, but kept a low profile until he was approached by Adam. The cyborg offered to have Spike's chip removed if he broke Buffy away from her friends, and lured her into the Initiative caves alone. He failed and was almost killed by Adam, but escaped and fought many of the demons captured by the Initiative on his way out.
Spike remained in his crypt, selling Buffy information, and grew more obsessed with her. Eventually Harmony came back, and he sheltered her in exchange for physical favors. When Buffy approached him to find Riley Finn and bring him to a doctor who was part of the Initiative, he and Harmony kidnapped the doctor and tried to force him to remove the chip. The doctor bluffed and pretended to do so, but Spike learned it was still in place when he was unable to attack Buffy.
His obsession with Buffy grew until it changed and became a sexual obsession. He began stalking her and hanging around outside her house. He later helped prove that Tara wasn't a demon. Soon after, Spike was hired by Buffy to tell her about the Slayers he killed. He told her his life story, and revealed to Buffy that each Slayer has a death wish. He came on to Buffy and she told him that he was beneath her. He grabbed a shotgun and went to kill Buffy, but instead comforted her when he found her crying over her mother's brain tumor. Spike began following Riley around and found him going to a burlesque house and paying vampires to feed off of him. He revealed this information to Buffy, and later admitted to Riley that he was attracted to the Slayer.
After Riley left, Spike began to romantically pursue Buffy and trying to impress her by not feeding off of accident victims. He later broke into Giles' magic shop with Dawn Summers, and read out of Giles' journals that Dawn was the Key, accidently revealing it to her. Buffy beat him up and he accused Buffy of being at fault for not telling her the truth, which Buffy later agreed with. Spike later revealed his love for Buffy, which repulsed her. At the same time, Drusilla returned and helped him overcome the chip to feed off of humans. He captured Buffy and tied up Drusilla and tried to force her to admit that she had feelings for him, to which Buffy "spit in his face." In the end, Dru left him again, Harmony walked out on him, and Buffy stormed off and had an enchantment cast keeping him out of her house.
Spike later commissioned a Buffy robot to use for his own carnal desires, and as it was built, he helped Dawn acquire the means to resurrect the deceased Joyce Summers. He revealed that he'd liked Joyce because she didn't treat him like a freak. Later once, the Buffy-bot was complete, the minions of Glorificus saw the robot protecting Spike and deduced that he must be the Key, capturing him. Upon figuring out that he was not the Key after all, Glory decided to beat the identity of the Key out of him, but he never broke, and managed to escape. When Glory did learn that Dawn was the Key, Spike stole a motor home to evacuate Buffy, Dawn and their friends from Sunnydale. However, they were opposed by the Knights of Byzantium. Spike helped protect Dawn from the Knights, but failed to protect her when Ben turned into Glory and captured Dawn. Since he was no longer human, he was the only one to remember witnessing Ben change into Glory. Spike began to track down Glory, and with Xander, went to his seeming ally Doc's apartment, only to discover that he was a minion of Glory. They failed to kill him but discovered the secrets of the Bleeding Ritual. Buffy made Spike promise to protect Dawn, and he vowed to do so till the end of the world. In the final battle with Glory, Spike tried his best to protect Dawn from being bled by Doc, but was thrown off the sacrificial structure, and seriously injured. When Buffy sacrificed her life to restore the barriers between dimensions, Spike mourned for her.
After Buffy returned from the dead, she confided her trouble to Spike, eventually leading to a sexual relationship between the two. Spike came to enjoy the power Buffy's secrets- including the relationship itself- gave him over her. When Buffy backed out of the relationship (she said it was unhealthy for them both), Spike became desperate, and eventually made an attempt at raping her. Spike's regrets over the incident turned to anger at his situation, and he went to Africa to find a way to "give Buffy what she deserves." After enduring the trials given him by an ancient demon, Spike gained the ability to give her what she deserved- as he was granted his long-lost soul.
Returning to Sunnydale, Spike wandered Sunnydale in secret, mentally unhinged by his experience. Eventually, his fragmented personality was manipulated by the First, who wished to bring him to the side of evil again. While Spike eventually regained something resembling his old personality, he was brainwashed by the First into overriding his chip and killing on regular intervals. When this was discovered, Spike was brought into Buffy's home, mainly to keep an eye on him and use him as a conduit to the First. This made the First more determined to sway him, and it sent the Bringers to retrieve him. The First used Spike's blood to release one of the Turakhan, and used the "Ubervamp" to torture Spike for days. Spike clung to his love for Buffy, however, and lasted long enough to be freed.
Soon afterward, his chip began to malfunction, causing Spike random bursts of pain, until Buffy was forced to call the Initiative and ask them to remove it. Despite this show of compassion, she soon chastised Spike for his recent softness, and expressed a desire to see the old, more violent Spike back again. Spike returned to his old ways with gusto. Spike ultimately sacrificed himself to finally destroy the First's minions and the Hellmouth.
Nineteen months later, Spike was resurrected at Wolfram & Hart as a non-corporeal being, who could not stray too far from the firm's headquarters, much to Angel's chagrin. Eventually, Spike became corporeal again, and acted as a foil for Angel when it became evident that the Shanshu Prophesy may apply to Spike and not Angel.
Spike began working as a free agent protecting people when Lindsay McDonald approached him, claiming to be named Doyle and having visions. Bu this was all a ploy to get into Wolfram & Hart and destroy both Angel and the Senior Partners, but was defeated.
Spike then took a job at Wolfram & Hart, attending to dirty business, but travelled to Rome with Angel when they found out that Buffy was dating their own nemesis, the Immortal, only to realise that both needed to move on with their lives.
Eventually, Angel infiltrated the Circle of the Black Thorn in an attempt to destroy the representatives of the Senior Partners in a final battle. Before the fight, Angel told his team to take the day to themselves, live it as if it were their last. Spike went to a bar to read poetry.
With the members of the Circle dead, Spike, Angel and the other survivors met in an alley as the armies of the Senior Partners advanced on them. Rather than flee, they faced their attackers head on.