The son of a prince of Glamorgan in Wales (Great Britain), and later founder and abbot of the great monastery of Llancarvan, Saint Cado came to Armorica between the 5th and 7th centuries (the exact period is undetermined), like so many other monks from his homeland, driven out by the Saxon invasion and who had found refuge there. He settled on the island in the Etel River that bears his name, built an oratory, founded a monastery, and dedicated himself to the evangelization of the region. He had a great influence.

The construction of the one-hundred-meter causeway that connects the island to the mainland is attributed to him. This construction gave rise to the following popular legend: Lacking the funds to build it, but desiring a bridge, Saint Cado was one day visited by Satan. Satan offered him a deal: he would build him a bridge on the condition that he receive as a reward the first living being to cross it. Cado accepted. In one night, the devil, aided by his mother, built the bridge. In the morning, Cado released a cat onto it and chased it, forcing it to cross from one bank to the other. Furious, the builder wanted to destroy his work. But Saint Cado had had time to bless the bridge and engaged Satan in a battle whose traces are still borne by the nearby rocks, upon which the Calvary of Pen-er-Pont was erected. "Stop!" the devil cried to his mother, who was carrying a final load of stones in her apron. She dropped them into the current, thus forming islets that divide it into two branches at the narrow passage of Pont-Lorois.

The monastery of Saint Cado grew to a certain importance. One day, the island was invaded by pirates, who devastated it and drove Saint Cado out. He returned to his homeland, where he distinguished himself through his charitable works and was later consecrated bishop and died a martyr. The community was re-established in the 11th century after the Norman raids. After a period of decline, it was united with the Abbey of Quimperlé, before disappearing altogether. Today, only the chapel and a nearby house remain, owned by the parish and occupied by a vicar who served until the French Revolution. Saint Cado is greatly venerated in Brittany, and many villages, chapels, and fountains, such as the one in Ploemel, bear his name.

Below the chapel, accessible via the stone path built in 2004, lies the fountain dedicated to Saint Cado. This structure dates from the early 18th century but was restored in the second half of the 20th century. The small shrine is surmounted by a sculpted Celtic cross depicting a crucifixion, installed in 1990. The shrine is partially buried, and the basin is covered by a barrel vault. An inscription, now illegible, dates back to the 17th century and is visible on the facade. In the back wall, a niche with a simple bracket once supported a now-lost statue. At high tide, the fountain is flooded by seawater, which mixes with the fresh water from the spring.