1-8
: 1s set & pass LSh to dance round the outside of the set (1M
  anticlockwise & 1L clockwise) to meet behind 3s & dance in between
  them
9-16
: 1M+4s dance LH across **while** 1L+2s dance RH across, 1s turn LH 1.1/2
  times to end in original places
17-24
: 1s+3s Adv+Ret & dance 1/2 Ladies' Chain
25-32
: 1s+3s Adv+Ret & dance 1/2 Men's Chain
33-40
: 1s & 3s chase clockwise (Men leading) 1/2 way round set to original
  places, all couples turn RH

{{explanation}}
2s repeat (dancing with 4s), then 3s repeat (dancing with 1s) & 4s repeat (dancing with 2s)