This depends very much on your definition of "plot." Whilst this short story may lack a traditional plot formation of introducing us to some characters with a problem, seeing how that problem develops and reaches its climax and then how things resolved, it certainly does offer us with a chronological narrative that focuses on one bed of flowers and the men and women that walk past it. Note how they are described:
The figures of these men and women straggled past the flower-bed with a curiously irregular movement not unlike that of the white and blue butterflies who crossed the turf in zig-zag flights from bed to bed.
The story then eschews the traditional narrative format to present us with snapshots of the characters who go past this one bed of flowers, and Woolf is careful to compare these people to fluttering butterflies. The focus is on nature, and perhaps ironically it is humans who are presented as flittering past whilst the slow arduous movement of the snail remains the one constant focus of the text throughout the entire story. Woolf seems to be making a comment about the ephemeral and temporary nature of our concerns, memories and preoccupations, which are compared to the constancy of nature.