A group of old friends gather at a remote Scottish castle, one last weekend before Georgie's wedding. Down at the loch, they take a group photo — but what they see stops them cold.

Because there's a stranger behind them.

None of them saw him, and nobody knows where he went. They're miles from the nearest town. Where did he come from, and where did he go?

As the weekend unravels and terrible secrets come to light, it soon becomes clear that their perfect weekend is turning into a perfect nightmare. They're desperate to leave — but someone won't let them.