As doubts on both sides mount, the Mr Big evidence becomes crucial to the Crown's case.
David's story looks increasingly shaky. He can't immediately explain a two-hour gap on the morning he said he saw Brett. And he has repeatedly misled police during questioning.
The Crown is telling a powerful story about David's guilt.
But the defence story is starting to take shape too. It's surely enough to create reasonable doubt.
The Mr Big sting becomes crucial. Does it dispel that doubt?