Lina says Doon's father had told her that he was in the library. She has tracked him down because she has something important to show him.
Doon tags along with Lina backs to her home. Mrs. Murdo is watching Poppy, since Granny had gone to bed earlier.
Doon is intrigued by the box with its complicated latch, and as soon as he sees the message, he agrees that it looks important. He says that he's seen some closed off tunnels, though they're not near the river (the word "river" was there in the message).
Lina asks if she can come check it out with him, and he agrees to let her into the Pipeworks the next day after everyone leaves at quitting time.
Doon hands Lina a slicker and some boots when she arrives, and together they slog through some tunnels.
When they come to the closed-off tunnel, Lina can feel the doorknob to the locked room for herself (there's no light in that tunnel, so there's nothing to see).
Suddenly, they hear someone coming, They retreat to the lit tunnel and hide, looking around the corner and listening.
There's a scraping noise and a thud, followed by slow footsteps. A key turns, a latch opens, and at that point Doon and Lina try to take a peek.
They get a glimpse of someone's leg, a dark coat, and messy hair. Then he vanishes back into the darkness.
Lina and Doon leave the Pipeworks after that, discussing who the mysterious person might be and how he got a key. Both remain a mystery.