You’ll see this in a lot of YA fiction, TV series, etc – it’s easy and effective.
It’s the “hunt for the missing family member.”
One of the main characters has missing parents, or a mother or father, or a big brother or sister, or younger sibling that has gone missing. Maybe even an uncle (although uncles generally turn out to be the bad guy).
It’s easy to introduce and explain, and justifies having good, smart characters make stupid decisions. You need conflict, and the plot should be character driven, which means your characters sometimes need to do stupid, desperate things that aren’t very safe.
Why would they risk it?
Why would they press on when their friends tell them to stop?
Why would they throw their life and current relationships away?
To find the truth.
This is Mulder chasing his abducted sister.
It not only helps us understand the character’s devotion to the cause, it also helps give the character a motivation to take deliberate, if reckless, action, without losing the sympathy of the reader, which makes it very effective.
Strangely, so far in most of my fiction, I haven’t used this technique, which is a mistake. Most of my characters are only children. Nobody goes missing. They don’t have a burning quest to find them. My characters are usually motivated by external opposition and trying to protect loved ones.
But this doesn’t have to be used by a main character – maybe you need a marginal character or friend to do something to jumpstart the plot into action at the right time… the “missing relative” gives them a reason to do so which is honorable, without turning them into a bad guy.