An example system for isolating cement slurry from drilling fluids includes a tool configured for installation on a landing collar of a casing. The tool includes a base that conforms to an inner circumference of the casing and a tube that extends uphole from the base. The tube has a first borehole that extends downhole through the base. The tube is perforated to allow cement slurry to pass from the casing into the first borehole. A cementing plug is configured to seal to the casing uphole of the base. The cementing plug includes a second borehole to receive the tube. The cementing plug includes a covering that extends across at least part of the borehole and that is configured to break in response to contact with the tube.