SERIES FINALE: This show’s entire current season has been unspooling this weekend, in nightly chunks of multiple episodes that, taken together, amount to a weekend binge-fest of all of Season 3. It builds tonight to a climax asking a variant on the old time-travel conundrum: If you could go back in time and kill Adolf Hitler as a child, should you? The evil seed here isn’t Hitler… but in this sci-fi story, he’s the next worst thing. And tonight, with the protagonists and even his parents in hot pursuit, his past, and his future, finally catches up with him.