Past perfect is not strictly necessary here. Especially since you have extra words (and then one day) indicating the relation of time between the two events.
In situations like this had can serve to emphasize the "pastness" of an event.
Past perfect is not optional is when it is not referring to an action at a specific time. The following is excerpted from here:
Compare the examples below. Here Past Perfect is referring to a lack of experience rather than an action at a specific time. For this reason, Simple Past cannot be used.
Examples:
She never saw a bear before she moved to Alaska. (Not Correct)
She had never seen a bear before she moved to Alaska. (Correct)
Continuing to excerpt from that same source - note that past perfect can also express that "something started in the past and continued up until another action in the past."
Some sentences (like the second) will sound a bit "off" if you don't use past perfect here, and you should generally use past perfect when you mean this to be the most clear.
With Non-Continuous Verbs and some non-continuous uses of Mixed Verbs, we use the Past Perfect to show that something started in the past and continued up until another action in the past.
Examples:
We had had that car for ten years before it broke down.
By the time Alex finished his studies, he had been in London for over eight years.
They felt bad about selling the house because they had owned it for more than forty years.
Although the above use of Past Perfect is normally limited to Non-Continuous Verbs and non-continuous uses of Mixed Verbs, the words "live," "work," "teach," and "study" are sometimes used in this way even though they are NOT Non-Continuous Verbs.