The case starts at Berczy Park, by a cast-iron fountain where 27 dogs stare up at a gilded bone and one cat looks the other way. You'll answer a rhyming riddle from the fountain itself to earn your clearance into the Bureau.

In the south market, between the butchers and the sandwich queue, you'll decode a question with every vowel swapped for an X and find a date on site. This was Toronto's first city hall and its jail at once, and the basement cells flooded every spring for decades. A hundred metres away, St Lawrence Hall has no ghost at all, just a carved head to pick out of four doctored photos and a room where Frederick Douglass once spoke.

At St James Cathedral, whose spire was once the tallest thing in the country, the sign out front has been tampered with in your photo. Its archive holds the papers of Dr George Grasett, killed by the fever of 1847 after 24 days running the fever hospital. Across King Street, the grand King Edward hotel stands where a ten-cell log jail went up in 1798, and you'll spot what has vanished from its awning.

Courthouse Square and the 1852 Adelaide Court House come next, and you'll solve a photo puzzle at each. Two rebels were hanged here behind a stockade in 1838, and the old courthouse cells are now a restaurant's washrooms. At Metropolitan United, in a park of its own off Queen Street, a plaque gives back a word blacked out of your note. In 1888 this congregation started the campaign that closed Toronto on Sundays.

Up on Bond Street, the statue in front of St Michael's Cathedral is missing something in your photo. Bishop Michael Power laid the cathedral's first stone in 1845, nursed the famine emigrants in the fever sheds of 1847 and was buried in its crypt at 42.

A few doors along, Mackenzie House is the narrow brick home of Toronto's first mayor. You'll use a number code to pull a password out of its blue plaque. When the house went to the city in 1960, the inventory ended with two words: one ghost.

The app shows you four versions of Massey Hall's red neon sign, and only one is real. Churchill, Helen Keller and Billie Holiday all appeared here, and in 1953 five jazz musicians played a set people still argue was the greatest jazz concert ever recorded.

At Old City Hall you'll count the carved faces erased from a photo of the entrance columns. The architect fought the council over his fees for years, so he carved the councillors into the stone as gargoyles and spelled his own name across the frieze.

The file closes behind the Eaton Centre, in the yard of the Church of the Holy Trinity. Four photos of the pavement labyrinth each have a different centre, and you'll pick the true one. The church opened in 1847 on one condition from an anonymous donor: its seats would be free forever.