- Merlin rides down to Caerleon to meet Ambrosius. He runs into Uther first, who tells him what he already knows: Dad wants him to help defeat the Irish because they are Pascentius' allies.
- Uther tells Merlin that he needs his skill set on this attack because the Irish stronghold of Killare is made of stone. He needs Merlin the Engineer on this one.
- After Uther leaves him to go to bed, Merlin sees someone enter Uther's quarters. It's Keridwen. Gasp.
- Merlin goes on to meet with Ambrosius. He tells Merlin that the Irish have a new young leader called Gilloman, who just can't wait to get into his first major war.
- And Gilloman's lovely sister is engaged to Pascentius. So with Pascentius moving in from the east and Gilloman from the west, things don't look great for Ambrosius.
- But if Merlin can help take Gilloman down, things will look rosier—and the quicker the better.
- Ambrosius has extra plans for Merlin's other, supernatural skill set. Killare has a Dance of stones, like Stonehenge, inside its walls. Ambrosius wants Merlin to take the "heart of Ireland"—the most powerful of the holy stones set up inside Killare.
- Of course, Merlin says he will go. But there's something else: Ambrosius wants to talk about a memorial that Tremorinus was to build for him.
- But Ambrosius has other ideas: he mentions to Merlin that he traveled through Amesbury (his birthplace) to get to Caerleon. When Merlin hears this, he feels sick.
- Ambrosius reminds Merlin that Amesbury isn't far from the place of a famous massacre masterminded by Hengist. He'd like a memorial built for the dead Brits. But it would also kind of be for him, too.
- The plan: rebuild the Giants' Dance (a.k.a. Stonehenge), since the massive stones have fallen down.
- Merlin says that he's the man for the job. If he doesn't have the magic for it—and he doesn't—he's got the engineering chops. Almost.
- Ambrosius tells Merlin that he will be buried at Stonehenge, which he calls "the threshold of all Britain."
- Merlin is thrown into a prophetic trance by the bright stars in the sky. He tells Ambrosius that when he's lying dead under the stone, the whole kingdom will prosper.
- Merlin also predicts that a new and awesome king from Ambrosius' family line is coming. (Hint: he's talking about King Arthur.)
- When Merlin snaps out of it, he remembers nothing of what he said. Ambrosius tells him that Merlin gave him immortality. Totally creepers.
- When they say goodbye that night, Ambrosius asks his son if they will ever meet again. Merlin wonders at first why he's being so weird.
- Then Merlin realizes: his prophecy was of the king's death.