In Ancient Egypt, the day begins normally. Ra, the mighty god of the sun, rises in the east at dawn, rides his boat across the sky, and descends into the Underworld at dusk. All is well and good.
The next day at dawn, he does not emerge again. As one might guess, this is somewhat problematic.
Of course, the sun is necessary to prevent small annoyances like "freezing to death" and "eventual, inescapable starvation-" but only the fortunate will survive to die in the dark. Ra's might cowed demons and sustained the other gods; without him, monsters that feared the light of the day will walk free. Eventually, the entire Egyptian pantheon will be helpless stop them.
An epic task falls to twenty-four champions from all across Egypt: to venture into the Underworld, following Ra's path down the River of Night. If the sun god has been captured or detained, rescue him. If an enemy stands in their way, kill him or die trying. The odds are stacked against them, but the consequences of failure are far, far worse than death.
After all, if they're too slow, there may not be a sun left to save...