The poem opens with some rich description of a swampy area where flax (a kind of plant) grows. Heaney describes the flies buzzing, and how the sun beats down on the mucky soil. He pays particular attention to the slimy frogspawn (what eventually becomes tadpoles, then frogs). This sparks a memory for the speaker, and he begins to talk about how in school, his teacher had students collect the gooey frogspawn in jars to watch it turn to tadpoles as part of a science lesson about frogs.
Then we're snapped into the present. One hot, steamy and stinky day, the speaker follows the sound of croaking frogs to its source. He sees more frogs than he's ever seen amongst the frogspawn (no, this is not a scene from a horror movie). They're croaking and slapping in the flax dam. Not surprisingly, he gets grossed out—so much so that he freaks out and runs away.