Mutual wills are different to, though related to, mirror wills. Mirror wills are wills, usually between spouses, which are basically identical except that that the spouses’ functions and bequests with respect to each other in each will are reversed. Mirror wills may or may not be made pursuant to an agreement between the spouses that their wills are mutual wills.
Mutual wills are two (or possibly more) wills made pursuant to an agreement between the willmakers as to how their respective assets are to pass on their respective deaths. The willmakers must intend that there is an agreement between them. The agreement may be (and usually is) by way of formal, written contract or deed. But it may also be by way of oral agreement, and may be implied from surrounding circumstances.
The agreement is basically to the effect that each of the willmakers undertakes not to change her or his will without notice to the other, and agrees not to change her or his will at all after the death of the other. If the surviving willmaker breaches either of those obligations, she or he may be bound to hold the assets subject to the agreement on trust according to the terms of the will which she or he had agreed to make. The defrauded beneficiary (the person who would have inherited had the will not been changed) has the right to sue to enforce the terms of that trust. Of course, they will only do so if they suspect or know about the agreement. And they will only succeed if they can prove the agreement.
An agreement for mutual wills is easy to prove if there is a written contract to that effect stored with the wills. It is difficult, but not impossible, to prove if the agreement is oral. Best practice is to have a written agreement, to reference it in each of the wills, and to store a copy of it with each of the wills.
If you suspect that you were the beneficiary of a will that has been changed contrary to an agreement for mutual wills, there are remedies available to you, and you should seek legal advice from a specialist estates lawyer.