Mixed Reality scene isn't playing in Unity in my HMD after I hit the Play button

I followed this Getting Started guide:
https://github.com/Microsoft/MixedRealityToolkit-Unity/blob/master/GettingStarted.md

I run the Mixed Reality Portal, and after a few seconds the Cliff House launches in my HMD. As instructed in the link, I press the Play Scene button in the Unity editor. My Unity scene's camera moves with the HMD like it should, but I still see the Cliff House within the HMD. I'd like to be able to see my Unity scene in the HMD. Is there something I'm missing to get that functionality?