One plus one equals one.
'Us' is like a play in three acts.
The first act, which I call 'Visitors', is creepy as hell (nothing is scarier than people standing still and not uttering a word). The second act - I call this 'Chaos' - is decent, but some 'rescues' are a bit cheap and predictable. The third and final act, 'Revelation', is just as thrilling as the first one (especially the flashback 'puppet' sequences are pretty amazing).
Verdict: four stars.