Another way to cover an event that occurred before the story action starts, yet relates to the plot, is to use flashbacks. If its a somewhat long explanation, then a prologue works best. If it can be broken up into several short scenes, then flashbacks can work. Make sure you know how to introduce and then close them out, coming back to the present, by using past perfect tense.
In other words, to transition to the past, say something like “he’d gone to the movie” (past perfect) as opposed to “he went to the movie” (simple past). After that first sentence, switch to simple past until the flashback is over, then use past perfect again to alert the reader that the story is back in the present. If flashbacks are not introduced and closed properly, it can be very confusing to the reader and cause one of those “WTF moments” you want to avoid. In other words, it will throw them out of the story as they go back to try and figure out when something happened and whether they missed something.
Careful handling of such writing protocols is what labels you a professional versus an amateur.