The subject is taken from Ovid, Metamorphoses, 5:572-641. As the nymph Arethusa bathed in a stream, the river god Alpheus chanced upon her and fell in love with her. Terrified, she fled but was eagerly pursued by her suitor. In the present composition we see Diana reaching down to envelop Arethusa in a cloud, just as the nymph's strength is beginning to fail her, and transform her into a stream which runs underground.
We are grateful to Giancarlo Sestieri for suggesting the attribution to Garzi, based on photographs.