A 2002 Hong Kong SupernaturalMartialArts [[MartialArtsMovie action movie]], starring Cantopop starlets Gillian Chung and Charlene Choi as vampire slayers.

Reeve (Ekin Cheng) is the best of the best when it comes to slaying the vampires infesting Hong Kong. He's tasked with training Gypsy (Gillian Chung) a novice slayer, who's more interested in fangirling him than concentrating on her training. Reeve's {{Tsundere}} sister, Helen (Charlene Choi), attracts the attention of Prince Razaf (Edison Chen), a "sentimental" vampire who refuses to drink blood. Meanwhile, a European vampire, the Duke (Mickey Hardt), is on a quest for the magical stones that will unlock a supreme power....

Features an extended cameo by Creator/JackieChan, who [[WolverinePublicity ended up on the DVD cover of some foreign editions]].

Followed up by The Twins Effect 2 in 2004, a sequel InNameOnly.

!!''The Twins Effect'' provides examples of the following tropes:

* ActionGirl: Both Gypsy and Helen.
* AristocratsAreEvil: The Duke.
* BadassLongcoat
* ChekhovsGun: Razaf's alleged love for bananas.
* ChopSockey: Even includes a fight scene with Jackie Chan versus biker vampires!
* ChineseVampire: Strangely, averted. Reeve mentions that there aren't usually many European vampires in Hong Kong, but almost every vampire in the movie is Caucasian. It's worth noting that the Chinese vampires we do see, such as Razaf and Prada, don't seem very different from the Duke and his cronies.
* CombatStilettos: Averted. The female characters all wear reasonably practical-looking boots when fighting.
* CoversAlwaysLie: [[http://3.bp.blogspot.com/_tW5TunwL010/SUf-lYosDWI/AAAAAAAAArQ/J8JrWZODeIQ/s320/twins+effect.jpg The Korean DVD Cover]] makes the film look like an intense thriller focusing on Jackie Chan and Ekin Cheng.
* DistressedDamsel: Prince Razaf
* DoggedNiceGuy: Prince Razaf, unusually for a vampire romantic hero, wins Helen by being sweet and sincere, not with dangerous sex appeal.
* ElevatorEscape: Helen pushes Razaf (who is weak from lack of blood) into an elevator and turns to face their vampire pursuer. She's almost killed before [[spoiler:Razaf pushes the 'Doors Open' button and pulls her inside with him.]]
* FanGirl: Gypsy for Reeve.
* [[BoyMeetsGhoul Girl Meets Vampire Prince]]: Helen and Razaf's love is complicated by the fact that her brother is the most accomplished vampire slayer in the world, and he's a vampire prince. His sidekick Prada advises him to take up with a vampire woman to offset the CarnivoreConfusion that will inevitably result.
* ImpaledWithExtremePrejudice: [[spoiler: Duke by the combination of Helen and Gypsy.]]
* KryptoniteProofSuit
* ManicPixieDreamGirl: Helen has elements of this paired with her Tsundere-ness.
* MarketBasedTitle: Released as ''Vampire Effect'' in America, likely because most Americans would not be familiar with the Twins, who do not play Twins in the film.
* MerchandiseDriven: The film was pretty much made to cash in on the popularity of the Twins, among other artists associated with Emperor Entertainment Group.
* StakingTheLovedOne: [[spoiler: Gypsy taking out Reeve.]]
* {{Tsundere}}: Helen.
* UhOhEyes: The eyes of vampires glow bluer the more powerful they get. [[spoiler: Gypsy's eyes go black when she swallows some of the magical energy that transforms her into a 'semi-vampire'.]]
* VampiresAreSexGods: Mostly averted. Prince Razaf spends a great deal of time and energy wooing Helen.
* VampireHunter: Reeve and Gypsy. [[spoiler: By the end, Gypsy, Helen, and Razaf.]]
* VegetarianVampire: Razaf lives off blood-wine sent to him by his father, the vampire king, and refuses to attack humans.
* WolverinePublicity: Jackie Chan appears prominently on the covers of both the US and Korean editions of the DVD. His appearance is actually just a small cameo.