There's the kernel of a decent film in here. Certainly, I found the first half pretty intriguing. Unfortunately, it suffers from a couple of pretty major flaws, and they sink it pretty thoroughly in the second half.
But let's talk about the premise and plot first, before the problems.
A secret research project - ostensibly tasked with attempting to discover a way to transmit matter - has accidentally stumbled across time travel. Specifically, they can jump to a point 56 years in the future. I'm not sure how they know it's 56 years, given that they're unable to find anyone alive to tell them, but they do.
So yeah, they're a bit anxious about the whole 'no living people' thing, as you might imagine. They've scanned the radio channels without success, and travelled far enough to check out a couple of local towns, but both were abandoned.
The project team's reaction to this is to keep their discovery secret and attempt to restore the human race in the future. They plan to do this by recruiting teenagers (anyone over 20 who goes forward suffers kidney failure within days or weeks at most) and sending them into the future to be seed stock. In the mean time, they keep the authorities in the dark about what's going on. Why is not explained, but this is the early 70s so scepticism about 'the man' was probably just assumed.
Anyway, the project gets shut down - probably due to them not showing any results and giving the higher ups the run around. About a dozen of the teenagers manage to escape to the future, however. That brings us to around the mid-point of the film, where it seems to be setting up for an exploration of this strange future world: presumably a tale of whether they discover what happened, and so forth.
The film's not going to play out like that, though, and it's here that its two biggest flaws really start to undermine it. The first and most obvious is the rather ham-fisted eco-moral of the ending. I mean, I think the evidence for climate change is overwhelming, and that resource scarcity is going to be a big problem for humanity, but this still made me roll my eyes.
The second problem is the main character. I don't know if you've seen the article on "Pixar's 22 rules" of writing, but one of them is:
You admire a character for trying more than for their successes.
The main character of this film fails to try. Some of her failure is understandable - she goes through some rough stuff - but it makes it very hard to care much about her travails when the script gives you no reason to like her or root for her success.
An interesting start unfortunately comes to nothing, so I can't overall recommend this. A shame, since it started quite promisingly.