Okay, so basically the situation is this:
You want to commit a mortal sin. (lets say masturbation)
You get your house to yourself and you get ready like your going to do it.
You don’t masturbate.
There would be no sin involved if that happened (as long as you didn’t have any impure thoughts while getting ready) since you didn’t actually do anything.
Lets try another situation. Lets say you want to kill someone.
You plan out the where it would happen, how you would do it, what you would do with the body afterwards, ect.
You never actually kill them (or attempt to do so.)
In that one, the only sin would (probably) be your wrath towards that person. Not murder.
So yeah, i’m pretty sure that if it’s a situation like those, then the person didn’t commit the sin they were planning on.