You'll start at the sailors' chapel on Rue Saint-Paul, where the app opens your case file and introduces the agent running it. Your first task is to solve a riddle with a word carved above the door. You'll learn about Jeanne Le Ber, who lived 19 years by choice in a cell behind its altar.

You'll walk to Bonsecours Market and find out which architect began the building. You'll learn how a market became the home of Parliament after a mob burned the real one down in 1849.

You'll climb Place Jacques-Cartier to Nelson's Column and spot what's missing from your photo of its pedestal. You'll learn why the city's oldest monument honours an admiral who never came within 3,000 miles of Montreal.

You'll stop at Château Ramezay and pick the right photo of the house out of four lookalikes. You'll learn about the many lives of the governor's mansion, from Benjamin Franklin's bedroom to an anatomy school.

You'll reach Montreal City Hall, decode a scrambled instruction and carry it out at the front of the building. You'll learn about the 1922 fire that left only the outer walls standing, and the city records that went with the rest.

You'll cross Place Vauquelin and spot what's wrong with your photo of the statue. You'll learn that the cells of the city's first British gaol are still under the paving, never filled in.

You'll walk out onto the Champ-de-Mars, beside the uncovered old city wall, and count the differences between a street sign and your photo. You'll learn why this slope was once called the best place in the British Empire to watch a hanging.

You'll stop in Place d'Armes and find what's hidden under the sticker on a nearby statue. You'll learn about Marie-Joseph Angélique, tried in 1734 for a fire that none of the 20 witnesses saw her set.

You'll look up at Notre-Dame Basilica from its steps and find what's missing from your photo of a statue on the front. You'll learn why its Protestant architect converted on his deathbed, and why he is the only person buried inside.

You'll find the corner of Rue Saint-Sulpice and Rue Saint-Paul and solve a riddle about the two saints whose streets cross there. You'll learn about Jeanne Mance, who opened the city's first hospital on this spot with eight beds.

You'll reach Pointe-à-Callière, where the city began, and find what's hidden under the sticker on a sculpture in the square. You'll learn about Montreal's first burial ground, and the frost that kept bringing its dead back up.

You'll finish on Rue Saint-Pierre at the old General Hospital, where the ruins of its first chapel lie open at ground level, and complete a riddle with a word you'll find on site. You'll learn about the fever ships of 1847, and the nuns who took turns nursing the sick until too many of them had died.