Back at Basil's flat,

The detective lit a lamp and was studying the list with his magnifying glass, Maria and Dawson stood at his side.

Basil: Offhand, I can deduce very little. Only that the words are written with a broad pointed quill pen which has spattered, twice. That the paper is of...

He tosses it in his hand, testing the weight to determine what type of paper it is.

Basil: ...native Mongolian manufacture, no water mark. And has...

With these words, he has discovered something else; Basil puts the paper to his lips and smacks it three times.

Basil: ...been gummed, if I'm not very much in error...

Basil sniffs at the paper, and holds it at arms length in disgust at the result.

Basil: ...by a bat who has been drinking Rodent's Delight! A cheap brandy sold only in the seediest pubs.

Dawson: Hmm. Amazing.

Maria: (smiling and awed) You are something Basil.

He blushes at that and smiled

Basil continues rummaging at his desk for the next stage.

Basil: Oh, not really, Doctor. We still don't know where it came from, and Thank you for compliment Maria

He pulls out his microscope to take a closer look.

Basil: Perhaps a close inspection will tell us something.

Basil focuses the lens on the letter.

Basil: Hmm-hmm. Hmm. Coal dust. Clearly of the type used in sewer lamps.

Maria tries to look through the microscope, but Basil took the list and was holding it over a small flame, letting it catch fire.

Maria: Uh, but Basil, I-I-

Basil: (Quietly) Shhh! Don't speak!

Basil lets the ashen remains of the list fall into a bowl, and pats it down with a small wooden masher. He pours the contents onto a glass jar of a yellow chemical, which turns blue. Maria and Dawson stare at the liquid while Basil returns with a vial of a red chemical.

Basil: Excuse me

He holds the red chemical over the jar carefully.

Basil: Steady hand...

Basil lets a single drop fall. A small puff of smoke comes out as the chemical turns violet. Basil sets the jar below a glass spout and reaches to the other side of the chemistry set to turn on a small flame.

The green chemical inside bubbles up and slowly makes its way through the tubes, Basil encouraging it soflty.

Basil: Yes, yes. Good, good. Come along, come along, come along, come along. Come along, come along, come along come along.... Haha... Yes, yes, good, good. No, bad. Good, good, oh, no. Come along, come on. Yes, come on.

Basil, Maria and Dawson watch intently as a single green droplet is hanging above the violet chemical.

Basil: Yes... Yes...

The drop falls, making the chemical turn red, to Basil's elation.

Basil: Ah-ha!

He puts his arm around Dawson in companionship, and kissed Maria on the cheek, to which she blushed at.

Basil: We've done it, my friends!, This reaction could only have been triggered by the paper's extreme saturation with distillation of sodium chloride.

Basil moves away from the table as Dawson continues to study the now-clear chemical.

Dawson: Salt water? Great Scott.

Basil rummages through his set of maps of the London area.

Basil: It proves beyond a doubt, this list came from the riverfront area.

Basil uses his darts to pin the map to the wall.

Dawson: Ah, now steady on there, Basil.

Basil: No, no. Elementary, my dear Dawson. We merely look for a seedy pub at the only... Basil pins the spot on the map with another dart "spot..."

Basil: (Lower voice) ...where the sewer connects to the waterfront.

Maria saw the map

Maria: (Lower voice) I know the place.

This caught Basil's attention, and turned to her.

Basil:(Lower voice) Where?

Maria lifts her head and stared at him with a determined look

Maria: (Lower voice) The Rat Trap.