WordPress password as plain text in email

No, WordPress passwords are not stored in plain text. They’re stored as hashes generated by an old version of PHPass, which I think does multiple rounds of salted MD5 hashes. You cannot extract the original password from this, by design.

See Why shouldn’t I store passwords in plain text? on Information Security StackExchange.

If you really really must do this, and I really can’t see why you should, the web request that performs the login will have the user’s password available in clear that you can use there and then. But there is absolutely no reason to tell the user the secret string they’ve just entered in your login box because they already know it don’t they?