"Nightsisters" represents the twelfth installment within the third season of the Star Wars: The Clone Wars animated television program. Its original broadcast occurred on January 7 of 2011. In the chronological sequence of the series, this episode follows Senate Murders and comes before the subsequent episode, Monster.
"Concerned about Asajj Ventress' increasing strength in utilizing the dark side of the Force, Darth Sidious instructs Count Dooku to eliminate her. Nevertheless, Ventress endures Dooku's attempt on her life, and the scorned former apprentice pledges vengeance, seeking the assistance of her female relatives -- the enigmatic Nightsisters -- in her malevolent plot.
Mother Talzin, who leads the Nightsisters, conceals Asajj and her fellow assassins with an invisibility field, allowing them to infiltrate Dooku's residence on Serenno. Although they are unsuccessful in assassinating the Count, their utilization of stolen Jedi lightsabers misleads Dooku into the false belief that the Jedi have attempted to murder him. Seeking protection, he requests a new apprentice from Mother Talzin."
Asajj Ventress spearheads a space battle in the Sullust system against the Republic forces, which are under the command of Jedi General Anakin Skywalker and Obi-Wan Kenobi. Ventress singles out Kenobi and launches an attack, causing damage to his starfighter, but Skywalker then retaliates, rendering her unconscious. Both parties crash within the hangar bay of the Separatist flagship.
However, a scheme to end Ventress' life is secretly unfolding. Darth Sidious engages in a discussion with Count Dooku, expressing worries concerning Ventress' escalating power, and directs his apprentice to terminate her before she becomes unmanageable. Dooku, bound by his master's will and fearful of defiance, reluctantly complies. When Ventress contacts him for assistance, he abandons her, citing her supposed failure. Enraged and betrayed, Ventress fiercely clashes with Anakin and Obi-Wan in lightsaber combat. The conflict is cut short when Dooku orders the flagship to be bombarded by the other Confederate ships to ensure her demise; the combatants narrowly escape with their lives. Subsequently, Dooku informs Sidious of Ventress' presumed death. The Dark Lord is assured that Dooku's loyalty remains intact.
Ventress is left adrift in space within her severely damaged fighter until a scavenging crew discovers her, intending to sell her back to her employers for a profit. However, Ventress eliminates them all, seizes their vessel, and travels to the location of her former order, the Nightsisters, where Clan Mother Talzin provides her with medical attention. While delirious with fever, Ventress experiences flashbacks of her rescue from a slaver by Jedi Knight Ky Narec as a child, after her mother sold her to him. She remembers her training under him, his death, her fall to the dark Side, her recruitment by Dooku, and his subsequent betrayal. Upon regaining consciousness, she vows to seek revenge against her former master. Mother Talzin assigns two of her most skilled warriors, Naa'leth and Karis, to assist her and equips her with the means to approach and fight Dooku, utilizing mystical invisibility potions and poisons.
The three Nightsisters journey to and infiltrate Castle Serenno on Serenno, finding him asleep. Dooku's senses are dulled by Ventress' poison dart and the cloak of invisibility surrounding his attackers, placing him at a disadvantage. Despite this, he fiercely retaliates, mistaking his assassins for Jedi because Mother Talzin has armed Ventress' team with lightsabers. On the verge of defeat, Dooku pretends to be completely drained of energy, but instead, channels his rage, unleashing a powerful blast of Force lightning, forcing them out of his palace and disrupting their invisibility. Mother Talzin is satisfied, believing that this assassination attempt will make Dooku more eager to find a new assassin and bodyguard for protection. She then contacts the Count and proposes a replacement—a male Zabrak warrior from her people—an offer Dooku decides to accept.