Occam's Razor suggests not necessarily the simplest explanation (there's nothing simple about how nuclear weapons work, for instance), but if you had two theories, you ought to pick the one that multiplies causes the least.
For instance,
1) The plane was hijacked by aliens
vs.
2) The plane was hijacked by zombies hidden in the cargo hold, then captured by aliens
They might both turn out wrong, but 1) is more a more economical theory, it's more likely necessary and sufficient if you had some evidence about either scenario.
