Premise: (spoiler… as if it mattered) A team of four spacemen (actually, two space men and two space women) return from space after 40 years in deep sleep on a mission to look at some alien probe a ways off. Arriving back on earth they are enslaved and taken to the local saw-mill to be put to work. Two of them are killed in the process.
It turns out the world has been taking over by alien “mites†which look vaguely like giant humanoid grass-hoppers. The mites killed off most of humanity with neutron-type-bombs and enslaved the remainder to help them deforest the place and ship the wood back to their homeworld for a tidy profit.