I have come across it in the 11th episode of the 10th season of Friends. Here is the context:
Rachel: (finishing the last of her drink) I am soo not going to do good on my SATs tomorrow.
Chandler: Well maybe if you go to school here next year we can totally hang out.
Rachel: (sarcastic) Oh yeah. There is a plan! Why don't I just start taking my smart pills now?
Does it mean that the plan is good?
Yes, it means the plan is good. I suppose “there is a plan” acknowledges that you see it and are therefore giving it recognition as being important.
Of course, as you’ve pointed out, this is said sarcastically, and so really means the opposite of that.
If you ignore (sarcastic) at the front, then you are correct that "There's a plan" (as it would normally be said/written) means something close to 'good' - it's only a good plan if it works!
However, the entire sentence is spoken sarcastically, so that needs to be negated - Rachel is actually saying that she doesn't like the idea at all.
There is a plan.
Sometimes you simply want to say something exists. To be means to exist (among a lot of other things), but you can't say:
A plan is.
and be easily understood. This is because to be is a copular verb, and used that way so often that, while the above sentence is technically correct, most listeners/readers will be expecting you to say A plan is X and will be waiting for the X.
So you can say There is X to say that X exists, when you don't want to say X is Y.
Why not use the word exists? You can, but for some reason it tends to sound weird in everyday speech except when talking about the nature of something. There is X is more idiomatic in everyday speech.
There is X doesn't automatically mean X is good. For example, there is danger, let's turn back wouldn't make "danger" a good thing.
