A MitM will create a certificate whose signature will be successfully validated against fake CA in your trust store.
for example, someone managed to install fake CA certificate (which is under attacker's control) into trust store, issued a fake certificate to security.stackexchange.com and routed you to fake StackExchange web site. Browser will be happy and tell that everything is just fine, while in reality you are visiting a fake web site, with fake certificate which is trusted by your browsed because you trust fake CA.