The free trial membership is absolutely free and will automatically expire at the end of the trial period. We don't collect any credit card information ahead of time, so you cannot be charged if you choose not to continue with Mystery Science.
To see when your free trial ends, you would click on your name at the top right side of the homepage, and then "Account & Membership". From there, you will be able to see your membership status along with the end date.
We hope you enjoy using your trial membership until then and will tell your friends and colleagues about us!