The Wave by Algernon Blackwood

Then, without any warning, he received a second shock -- it fell upon him from the blue and came direct from Lettice.

The occasion was a tennis party in the garden by the sea where the family had come to spend the summer holidays. Tom was already at College, doing brilliantly, and rapidly growing up. The August afternoon was very hot; no wind ruffled the quiet blue-green water; there were no waves; the leaves of the privet hedge upon the side of the cliffs were motionless. A couple of Chalk-Blues danced round and round each other as though a wire connected them, and Tom, walking in to tea with his partner after a victorious game, found himself watching the butterflies and making a remark about them -- a chance observation merely to fill an empty pause. He felt as little interest in the insects as he did in his partner, an uncommonly pretty, sunburned girl, whose bare arms and hatless light hair became her admirably.