Four steps: write a story, tag the words you want blanked out, hand the link to someone else, and they fill in the blanks without ever seeing the story.
Blanks can share an answer, so a character's name only has to be supplied once. Tag a repeated word and you'll be offered the other mentions in one click; to link words that aren't identical, tag the first one, then select the second and pick it from the "reuse an answer" row.
There is no server. Every step is packed into the page's query string as base64, so whatever you're looking at is what a shared link will show. That does mean the story text is inside the link you send to your friend - it's hidden from them in the page, but it isn't a secret. Don't put anything in here you'd mind them finding.
Links get long (a 300 word story lands around 2,000 characters), which is fine for messaging apps but may be worth shortening for anything printed.