This is a common misunderstanding. Your sentence mentions an action. That action seems to be in the past. The statement uses the present tense. Why?
Some verbs express action. Other verbs express state of being*. The verb "to have" expresses a state of being.
1) I saw the film.
2) I have seen the film.
3) I had seen the film.
In 1), the only verb is a past-tense action verb.
in 2), there are two verb words which form a single verb phrase. The "have" is a present-tense state-of-being verb. The "seen" is a verb without any tense but with the perfect aspect. This form does not express the action of the verb "to see". It expresses the result of that action.
I have seen the film. I possess a result of the action.
In 3), the "had" is a past-tense state-of-being verb.
The structure of 3) is rarely used on its own. Often, it's used to suggest that one thing is even further in the past than another:
4) I had seen the film when I read the book.
The action of "to read" is in the past. At the time of the action of "to read", my state of being included the result of the action of "to see". This suggests that that action of "to see" happened before the action of "to read".
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* As far as I can tell, we use the phrase "state of being" because the word "condition" would be confusing. Verbs like "to be" and "to have" express a condition in the sense of "the way things are", rather than the sense of "something required for something else". We use the word "condition" in the second sense when discussing subordinate clauses, so we use "state of being" when discussing what a verb can express.
In English there are two different voices that may be employed when making a statement: active voice and passive voice.
In the active voice the subject of the sentence is also the agent in the sentence; the subject performs the verb's action on the patient.
In the passive voice the patient becomes the subject of the sentence, getting acted on by the verb. A sentence in the passive voice uses the auxiliary verb be to indicate the tense and the main verb always becomes the past participle.
In your extract, the one is not the agent (the one doing the seeing), but is the patient (the one that is seen), so it is in the passive voice and uses the past participle form of the verb see. I.e. the first extract is the correct one:
.....the one seen at the beginning is ....
Best Answer
Typically we use the past simple (saw) for events in the past and the past perfect (had seen) for events further in the past. For example, you could say:
You'll need to give us more context for these sentences to know which one would be more correct, because they can both be used in different situations.