Trapped in a simulated loop of Earth’s final year before war ravaged the world, a increasingly pessimistic time traveller-to-be must test an increasingly severe sequence of events before going back in time to save the planet for real.

Having travelled to the future and seen the end of humanity, a misanthropic scientist has one year to change events that lead to world war 3.

misanthropic seem to be a better and shorter description than increasingly pessimistic.
the fact he is in simulated loop is just an artefact of the plot, so I left it out of the Logline.

The original Logline lead me to more questions:
is he really trapped in the simulation?
if it is the loop what is the time constraint?
what and who is the antagonist?

>>>back in time

Back how far?

>>>back in time

Back how far?