Complex sentence: Then he returned back to Laura's time where he saw Laura being killed by Kokoschka and his men so he zoomed back quickly to 1944 and did some calculations to arrive at that place a little earlier.
Split into simpler sentences: Then he returned back to Laura's time, but there he saw Laura being killed by Kokoschka and his men. Stefan quickly returned to 1944 and did some calculations to arrive at that place a little earlier.