If the defendant makes the plaintiff a written offer, and the defendant fails at defense, offers a specified sum of damages
If plaintiff accepts within 10 days, and the plaintiff prevails, proper notice given then the damages will be assessed based on the offer of settlement.
If plaintiff doesn't accept, you cant talk about the offer in the trial.
If plaintiff doesn't accept and plaintiff's damages awarded are not greater than the settlement offer, then neither party may recover costs.
Wis stat 807.01(2)