Get out the microscope, because we’re going through this poem line-by-line.
From the bank and from the river
He flashed into the crystal mirror,
- Now he shows up in the Lady's "crystal mirror." She finally sees this superman we've already heard so much about, and we have to believe she's impressed.
"Tirra lirra," by the river
Sang Sir Lancelot.
- Lancelot is singing a song as he trots along, and we get a little snatch of it, just the words "Tirra Lirra."
- This may be a reference to Shakespeare's The Winter's Tale (Act 4, Scene 3) where one of the characters sings a song about "The lark, that tirra-lirra chants." It's probably also just a nonsense word from an old song, like "hey nonny nonny" or "sha la la." It's important, however, because it echoes the Lady's singing from earlier in the poem.
She left the web, she left the loom,
She made three paces through the room,
She saw the water-lily bloom,
She saw the helmet and the plume,
She looked down to Camelot.
- When the Lady sees him, she makes a fateful choice. She steps away from her loom and walks across the room. For the first time she actually looks outside, and sees the real world, the lilies, the knight's helmet, and Camelot.
- The poem doesn't actually say that she's fallen hopelessly in love at the very sight of Lancelot, but that's pretty much the implication.
Out flew the web and floated wide;
The mirror cracked from side to side;
"The curse is come upon me," cried
The Lady of Shalott.
- Of course we learned early in the poem that the Lady is forbidden by the mysterious curse from looking outside. So when she does, her web flies apart and the magic mirror cracks.
- The Lady realizes right away that she's in trouble, and the third part of the poem finishes with her crying out: "The curse is come upon me."