What if...

What if an immortal man were to build a time machine to save the Earth before it gasps it's last breath?

The main character(s) is(are) Jon Smith.

Jon creates a time machine, invented by his parents and travels back in time to create six devices that will, when his quest comes full circle, save the Earth. The machine requires one key to start, as does each device. These keys are those who inherit the place they are hidden in (Key Inheritance)

What if the machine and/or any/all of the devices are discovered before the set time?

What if the keys are taken?

What if the devices are not constructed in time?

What if there are other time travelers?

What if one 'key' refuses the responsibility?

The mind boggles...