The novel opens when a stranger arrives at Iping and no one knows what to make of him. From the very beginning we know that there's some weirdness here, but we're not quite sure what. (Unless, of course, we've seen the title of the book we're reading. Oh well.)
The stranger doesn't fit in well in Iping and everyone's pretty suspicious of him. This makes it tough for him to just live his invisible life. Starting with a robbery and ending with the Invisible Man revealing himself, there is constant discord.
At first, the conflict is just between the Invisible Man and the village of Iping (at least, as far as we know). Soon, though, the Invisible Man brings Thomas Marvel in as his accomplice, which just complicates things. It causes more trouble in town, and also leads to bigger problems for the Invisible Man, since Marvel doesn't want to help him. So now, the Invisible Man is fighting the villagers and his ex-accomplice. Complicated, indeed.
a weird way, the climax of the story takes place in the past. It's all
about the Invisible Man's development of his invisibility formula. It
includes him stealing from his father, burning down his boardinghouse,
almost being caught by the Salvation Army, breaking out of the
department store, and realizing that being invisible isn't so great.
This is also the section where we finally learn the Invisible Man's
name. So, if this story is a mystery about the stranger, we finally
unravel the mystery here.
After Griffin invents the invisibility formula and finds Kemp, it seems like everything will go okay. (Well, okay for Griffin – not for everyone else whom he plans to murder and terrorize.) But when Dr. Kemp betrays the Invisible Man, the IM tries to murder Kemp in response. Now it's Invisible Man vs. the world. The battle has begun, and we can't wait to see how it ends.
Spoiler alert: the world wins. The Invisible Man gets killed by some workmen and slowly but surely, Griffin becomes visible again. This is the big reveal of the book, though it reveals info we already knew.
After Griffin dies, Marvel still has his scientific notes, but since they're totally in code, he can't really put them to use. Maybe whoever inherits those notes will be able to recreate the formula or some other scientific miracle. It's both a closed conclusion – because Griffin is dead – and an open conclusion – because who knows what Griffin's notes might lead to next.