Psycho Beach Party
(2001)

Starcat (Nicholas Brendon) dumps Marvel Ann (Amy Adams) and gets together with Chicklet (Lauren Ambrose).

Chicklet, having relived the traumatic memory of accidentally killing her younger brother, is cured of her multiple personalities.

Provoloney and Yo-Yo finally admit their attraction.

Capt. Stark and Kanaka reunite.

Lars is revealed as the murderer.

All is well...

Until it turns out that Chicklet's alternate personality, the world-domination intent dominatrix Ann Bowman, is alive and killing people.

Thanks Mary C!