Think of it almost in superhero terms. Basic background is that waaaaaaaaaaay back when, the various Gods locked the titans away. They were for killing them, but found out that was a bad idea. IIRC, they killed Ymir, and that caused the last ice age. Rather than risk destroying the entire world, they locked the titans away .. elsewhere.
The titans want to get loose and likely take revenge on the Gods, destroy the world in the process and be generally unpleasent. As a mortal son or daughter of one of these deities, you're the front line to keep the titans locked away. You have your own supernatural speed, strength, charisma and so on, and relics that allow you to channel specific powers.
My pen and paper group is playing a Scion campaign at present. In the first episode, the characters were drawn together .. my own Scion of Sun Wukong (Chinese), a Scion of Thor (Norse pantheon), a Scion of Anubis (Egyption pantheon), and three Scions from the Greek pantheon (Hermes, Dionysus and Athena I think).
They found out there was a titan buried under a Romanian castle, and had to go destroy a device which could be used to wake it. They realised that the one that gave them their task was actually titanspawn, and the device was what was keeping the titan contained. They went back out, fought a fire giant and wrapped the situation up.
From there, a brief jaunt over to Dublin for some personal business of the Scion of Thor. In Dublin they were attacked by Yakuza, which led them to Las Vegas, where they spiked an Onis plans to get his hands on a powerful relic, defeating an Aztec monster to gain it for themselves.
From there, they're off to California to investigate an old mining town which was apparently closed down years ago ... which leads to the question of who sent them the fax using the stationary from that mining company?
As the campaign progresses, the players will grow in power, becoming demigods and finally Gods in their own right.