After defeating the Israelites, the Philistines take the Ark of the Covenant. They place the Ark in the temple of their god, Dagon. This Philistine god is widely believed to be a fish god. This makes the most sense considering the Philistines were mainly sea-faring people (1-5).
Because the Philistines took the Ark, God plagues them with a terrible disease (tumors) that causes bleeding in some uncomfortable places. The leaders of the Philistines gather together and decide to send the Ark back to the Israelites (6-12).