Directorik has given you excellent advice. Whether a series or set of sequels, you need to plan your 'hooks'. For a series, every episode has three story lines: the series story, the season story and the episode story. For a sequel set (trilogy), the hook is what carries us into the next story. However, each episode or feature has to tell its own complete story. Though with some television episodes, the major plot is so large, it warrants a "to be continued", that really annoys audiences if you try that with a feature.
As such, you should always have three story lines in your script: tie-in, central, and unfinished. The central story is always complete. That's your feature. Below is how I often visualize it, though the actual placement in the script can vary. This uses a three act model approach.
Code:
Feature:
A1a ... A1b ... A2a ... A2b ... A3a ... A3b ...
B1a ... B1b ... B2a ... B2b ...
C3a ... C3b ...
Sequel Territory:
B3a ... B3b ...
E1a ... E1b ... E2a ... E2b ... E3a ... E3b ...
D1a ... D1b ... D2a ... D2b ...
Prequel Territory:
F1a ... F1b ... F2a ... F2b ... F3a ... F3b ...
C1a ... C1b ... C2a ... C2b ...
For reference
1a - Set Up (introduce the characters and situation)
1b - Introduce a New Situation and a Complication
2a - Formulate a Plan of Action to Succeed (character throws self in headlong)
2b - More Complications and Higher Stakes (usually a major setback)
3a - Final Push to Succeed with it looking unlikely (suspenseful moment, climax)
3b - Resolution and the Ever After