Christopher comes up with the idea to build a cannon in the apothecary shop where he is an apprentice. However, he has difficulty getting his friend Tom's attention. Tom is preoccupied by an imaginary battle with the shop's stuffed bear. Once he realizes what Christopher wants, Tom is against the idea, referencing unsuccessful adventures they've had in the past. It would be illegal, and Lord Ashcombe, King Charles’ Warden, is on the streets hunting for a killer of apothecaries. Nevertheless, Master Benedict, the owner, will be gone for a long time, and Christopher convinces Tom that the two should make gunpowder.

Christopher takes out a coded recipe for gunpowder. He explains that apothecaries need to keep their recipes secret and that Master Benedict has been teaching him different ciphers. The gunpowder recipe is written in a substitution cipher. Christopher decodes it by replacing the numbers with letters. Then, Christopher grinds charcoal while Tom crushes saltpeter. They then use a piece of pipe as a cannon and select lead shot instead of a cannonball. Afraid of using it outside and alerting the neighbors, the boys instead shoot at an iron cauldron. Christopher lights the fuse. The cannon explodes, and the pipe hits him between his legs.