The Lake Lausanne episode belongs to the Vulgate Merlin continuation and takes place during Arthur's continental adventures. Merlin tells the king that a monstrous cat has devastated the country beyond the lake. The adventure is not a Lady of the Lake story and has no underwater court. The lake matters because it is the place from which the creature first enters human hands.

The origin begins on Ascension Day. A fisherman promises God the first fish he catches, then keeps a valuable pike rather than surrendering it. He breaks the promise again when the second catch is even better. On the third cast the net contains not another fish but a small black kitten. The sequence makes the animal's arrival the consequence of a broken sacred vow.

The fisherman takes the kitten home because he thinks it will catch rats and mice. It grows to enormous size, kills the fisherman, his wife and his children, then takes refuge on a mountain beyond the lake. The surrounding country becomes unsafe, and people abandon or avoid the district.

Arthur's fight with the cat is unusually physical. The animal attacks his face and chest and drives its claws into his shield. Arthur cuts off its forelegs, then its hind legs, and finally opens its body with his sword. The victory is costly rather than effortless, but the monster is clearly killed in this French version.

After the battle Arthur orders the mountain formerly called Mont du Lac to be called Mont du Chat, the Mountain of the Cat. The place-name explanation belongs to romance, not to verified historical etymology. It gives the landscape a memorial of Arthur's feat in the same way many medieval stories attach heroic actions to real or imagined landmarks.

The geography does not fit neatly. Lake Lausanne is conventionally taken as Lake Geneva, on whose northern shore the city of Lausanne stands. Mont du Chat, however, is a real mountain in Savoy beside Lac du Bourget, well south of Lake Geneva. Modern scholarship therefore treats the episode as a mixture of Alpine place-names rather than silently relocating the whole narrative to one lake.

The story resembles the Welsh Cath Palug tradition because both involve a small cat associated with water and human nurture before monstrous growth. The French text, however, gives a different origin through the fisherman's broken vow and has Arthur kill the creature. It never names the cat Cath Palug or the Sons of Palug. Similarity and later continental development should be acknowledged without making the two stories identical.