Lancelot, known as the Knight of the Lake, had been loyal to Altria as a Knight of the Round Table. He was one of the few individuals that Altria viewed as a friend, and as such was privy to the secret of her true gender, as well as the nature and properties of her sword.
Lancelot's downfall began when he fell in love with Altria's queen, Guinevere. Though the king was actually a woman, she had married in order to maintain the pretense that she was a man and the image of a stable royal family. Guinevere, in turn, willingly sacrificed her own life as a woman in order to preserve this ideal government.
When Lancelot and the queen began their affair, the Knight of the Lake considered fleeing Camelot with her, but his loyalty to Altria and his sense of duty prevented him from doing so. When political enemies of the king exposed the affair, the queen was ordered to be executed. Left with no other choice, Lancelot attempted to save her, killing several of his fellow knights including Gawain's brothers, Gareth and Gaheris, in the process.
Lancelot, who was considered the perfect knight, Guinevere, the queen who had made her beloved betray his duty, and Altria, the king who blamed herself for her loyal knight's despair, all suffered from the resulting fallout.
Although Altria forgave Lancelot's actions, Gawain still bore great hatred towards Lancelot for his betrayal and the death of his brothers.
His disloyalty had fractured the balance within the Knights of the Round Table. This incident eventually became the trigger of civil war and carnage, and the country shattered to pieces in the flames of war.
Lancelot, even after being excommunicated, wanted to participate the Battle of Camlann to serve his King but Gawain hostilely rejected him which resulted in the downfall of his King and country.
When Lancelot heard the news of his King, Lancelot sank in despair and escape through madness thus, when he became a Heroic Spirit, he was summoned as the insane Berserker.
Berserker is summoned by Kariya Matou in the 4th Holy Grail War. Zouken Matou, deciding that Kariya was too lacking as a Mage, compensated by forcing him to obtain a Servant from the most powerful of the seven classes - namely the Servant of Mad Enhancement, Berserker.
During the War's first battle between the Servants, Lancer strikes Saber with Gáe Buidhe and attempts to finish her off, but their fight is interrupted by Iskander. In the ensuing chaos, both Archer and Berserker arrive and engage one another in battle. They are evenly matched, with weapons of Archer's Gate of Babylon being caught and effectively wielded by Berserker to deflect any other weapons thrown at him.
Archer is forced by his Master to retreat, but Kariya, unable to control his Servant any longer, is unable to order Berserker to do the same. As a result, Berserker turns his attention to the injured Saber, but is stalled by Lancer's interference. However, Lancer's Master, Lord Kayneth Archibald El-Melloi, decides that Saber is the greater threat, and uses a Command Spell to coerce his Servant into aiding Berserker. This order is quickly retracted when Iskander interferes again, attacking Berserker from behind and threatening to team up with Saber. Both Berserker and Lancer retreat soon after.
During the battle against Caster, Berserker takes control of an F-15 jet to use as his Noble Phantasm, and engages Archer in aerial combat at his Master's behest. Their fight is left unresolved when Berserker catches sight of Saber, and ignores Archer in favour of attacking the female Servant. He is defeated when Archer strikes him from behind with a barrage of Noble Phantasms. His Master, Kariya, is likewise defeated when he engages Tohsaka Tokiomi in combat.
Berserker is next sent out to kidnap Irisviel von Einzbern, in order to fulfill Kariya's end of a bargain with Kirei Kotomine. He uses his abilities to assume Rider's form, effectively fooling Saber and her Master into pursuing the wrong Servant. Saber engages the true Rider in battle, allowing Berserker to safely deliver Irisviel to his Master.
On the final day of the 4th Holy Grail War, Berserker engages Saber using submachine guns as his Noble Phantasm. (The anime depicts him with a Sniper Rifle and a Submachine Gun.) She manages to disarm him and damage his armour, but her killing blow is stopped short when Berserker deftly catches her blade between his hands. Saber realises that Berserker possesses knowledge of Excalibur's shape and length despite its invisibility, and deduces that he had known her when still alive. She demands that he reveal his true identity, to which Berserker responds by drawing his own sword, Arondight. His helmet finally shatters, revealing him to be Saber's old friend and greatest knight - Lancelot of the Lake.
Overcome by guilt of how she had contributed to his downfall in life, Saber loses her will to fight, and is brutally battered by Lancelot. She blocks one final blow from Berserker, expecting the next to be a killing strike, only to have him completely cease all movement. At the instant of his last blow, Berserker's Master, Kariya, had been completely drained of magical energy, effectively cutting off Berserker's source of power.
Taking advantage of the situation, Saber drives Excalibur into the frozen Berserker, dealing him a mortal wound. She tearfully tells him that she had wished to win the Holy Grail to atone for her past mistakes, including her part in Lancelot's death.
Having regained his sanity, Berserker admits that he wished Saber had condemned him for his crimes, and demanded penance of him. Instead, her willingness to forgive him made it impossible for him to forgive himself. Berserker collapses into Saber's arms, sighing that, in the end, he would die in the bosom of his king, like a loyal and just knight. Unable to sustain his physical form any longer, Lancelot fades away but he commented to Saber