I'm going to review this as the end of season three, rather than the end of the whole series although, if you asked me, I'd say that this felt very much like the end of the run.
With the invasion averted Bill Ward (Gabriel Byrne) goes to prison for Emily's murder. Various people around the globe begin to experience hallucinations of the other life, that they no longer had to endure. Zoe (Pearl Chanda) is unable to shake hers, and the visions of Bill she sees, so she goes to question him. Catherine (Lea Drucker) comes to believe that the visions are related to a blackhole that has formed outside of the Earth's atmosphere, a link between the timelines.
Whilst sharing little with the source novel other than a name, I felt the first season of "War of the Worlds" was alright and the second was an improvement, even if it remains a grim experience. I would say that I thought the show reached a perfect ending in the second season, and running this third was a risk. I'm not convinced that I was wrong, but this third run didn't undermine what we've seen before.
With the loss of some of the characters, most notable Daisy Edgar Jones's Emily in the last season, the story introduces two new characters, Richard and Juliet, played by Lukas Haas and Lizzie Brochere as astronauts on a space station orbiting the Earth. Though initially a confusing side story, they do eventually become integral to the season.
I suppose I feel about this season much the same I've felt about "War of the Worlds" since the beginning. It's an expansive, well made and wonderfully shot show that I perhaps appreciate rather than love, because of the grim aspects to it. If you are at all interested it's work taking the journey, but I can't see it winning many non-sci fi fans over.