Fionn mac Cumhaill, better known as Finn McCool, is an elite military commander, a secret outlaw raised in the woods, a poet with a magical thumb, and a giant who built a causeway to Scotland. This deep dive traces the layers of the legend from his father Cumhall's abduction of Muirne, the Battle of Cnucha, and the takeover of the Fianna by Goll mac Morna, through the boy Deimne's hidden upbringing under a druid and a warrior woman. It recounts the accident at the River Boyne where the apprentice burns his thumb on the Salmon of Knowledge that his master Finnegas had chased for seven years, gaining wisdom he can summon by putting his thumb in his mouth.

The episode follows the ten-year-old Fionn saving Tara from the fire-breathing Aillen, who lulled the city to sleep every Samhain for 23 years, by pressing a cold spear blade to his forehead ...