First occurrence: an action plan
Based on the results of work, an action plan shall be developed and submitted to the head of the executive committee
Second occurrence: the plan
The plan will have four sections.
Stylistically, in English, a general thing begins life with a/an. Then, the next time it is mentioned, it becomes "the x".
This is a standard style.
We're building a pool in our garden. The pool will measure 30 x 19 yards.
example for moving from "an action plan" to "the action plan"
This is an extremely useful usage example for Russian speakers, as there are no determiners in Russian.