A method for fluid treatment of a borehole including a main wellbore, a first wellbore leg extending from the main wellbore and a second wellbore leg extending from the main wellbore, the method includes: running a wellbore tubing string apparatus into the first wellbore leg; conveying a plug into the wellbore tubing string apparatus to actuate a plug-actuated sleeve in the wellbore tubing string apparatus to open a port through the wall of the wellbore tubing string apparatus covered by the sleeve; employing a plug retainer to retain the plug in the tubing string against passing outwardly from the tubing string apparatus; allowing fluids to flow toward surface outwardly from the tubing string apparatus; and performing operations in the second wellbore leg.